build: no-longer disable WARN_CXXFLAGS when CXXFLAGS is set

Now that CXXFLAGS are back in user control, I don't think there's a
reason to no-longer use our warning flags when CXXFLAGS has been
overriden (this includes when building from depends).

Anyone can suppress warnings from third-party code by
passing the relevant `-Wno-` options in CXXFLAGS.

Fixes: #18092.

Author: fanquake

configure.ac

@@ -401,37 +401,35 @@ if test "$enable_werror" = "yes"; then
   ERROR_CXXFLAGS=$CXXFLAG_WERROR
 fi
 
-if test "$CXXFLAGS_overridden" = "no"; then
-  AX_CHECK_COMPILE_FLAG([-Wall], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wall"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wextra], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wextra"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wgnu], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wgnu"], [], [$CXXFLAG_WERROR])
-  dnl some compilers will ignore -Wformat-security without -Wformat, so just combine the two here.
-  AX_CHECK_COMPILE_FLAG([-Wformat -Wformat-security], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wformat -Wformat-security"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wvla], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wvla"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wshadow-field], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wshadow-field"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wthread-safety], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wthread-safety"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wloop-analysis], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wloop-analysis"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wredundant-decls], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wredundant-decls"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wunused-member-function], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunused-member-function"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wdate-time], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdate-time"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wconditional-uninitialized], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wconditional-uninitialized"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wduplicated-branches], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wduplicated-branches"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wduplicated-cond], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wduplicated-cond"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wlogical-op], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wlogical-op"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Woverloaded-virtual], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Woverloaded-virtual"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wsuggest-override], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsuggest-override"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wimplicit-fallthrough"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wunreachable-code], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wdocumentation], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdocumentation"], [], [$CXXFLAG_WERROR])
-
-  dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
-  dnl unknown options if any other warning is produced. Test the -Wfoo case, and
-  dnl set the -Wno-foo case if it works.
-  AX_CHECK_COMPILE_FLAG([-Wunused-parameter], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-parameter"], [], [$CXXFLAG_WERROR])
-  AX_CHECK_COMPILE_FLAG([-Wself-assign], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-self-assign"], [], [$CXXFLAG_WERROR])
-  if test "$suppress_external_warnings" != "yes" ; then
-    AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"], [], [$CXXFLAG_WERROR])
-  fi
+AX_CHECK_COMPILE_FLAG([-Wall], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wall"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wextra], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wextra"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wgnu], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wgnu"], [], [$CXXFLAG_WERROR])
+dnl some compilers will ignore -Wformat-security without -Wformat, so just combine the two here.
+AX_CHECK_COMPILE_FLAG([-Wformat -Wformat-security], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wformat -Wformat-security"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wvla], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wvla"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wshadow-field], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wshadow-field"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wthread-safety], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wthread-safety"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wloop-analysis], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wloop-analysis"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wredundant-decls], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wredundant-decls"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wunused-member-function], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunused-member-function"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wdate-time], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdate-time"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wconditional-uninitialized], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wconditional-uninitialized"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wduplicated-branches], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wduplicated-branches"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wduplicated-cond], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wduplicated-cond"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wlogical-op], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wlogical-op"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Woverloaded-virtual], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Woverloaded-virtual"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wsuggest-override], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsuggest-override"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wimplicit-fallthrough"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wunreachable-code], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wdocumentation], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdocumentation"], [], [$CXXFLAG_WERROR])
+
+dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
+dnl unknown options if any other warning is produced. Test the -Wfoo case, and
+dnl set the -Wno-foo case if it works.
+AX_CHECK_COMPILE_FLAG([-Wunused-parameter], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-parameter"], [], [$CXXFLAG_WERROR])
+AX_CHECK_COMPILE_FLAG([-Wself-assign], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-self-assign"], [], [$CXXFLAG_WERROR])
+if test "$suppress_external_warnings" != "yes" ; then
+  AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy], [NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"], [], [$CXXFLAG_WERROR])
 fi
 
 dnl Don't allow extended (non-ASCII) symbols in identifiers. This is easier for code review.