Merge #16424: build: Treat -Wswitch as error when --enable-werror

fabfcb5 build: Treat -Wswitch as error when --enable-werror (MarcoFalke)

Pull request description:

  By default we set `-Wall`, which enabled `-Wswitch`, so this already prints warnings. However, it can be additionally be turned into an error when `--enable-werror` to be extra safe.

ACKs for top commit:
  practicalswift:
    utACK fabfcb5
  Empact:
    ACK bitcoin/bitcoin@fabfcb5

Tree-SHA512: f6bd6dba93a4f3740811eb338b6db93b4f72d237afe848aefd212abecaf4f430c5a417ccb2f9fec0bdbc46001176f0cfa0bbf4d99a7fcf0e34dca4a9476e8456

Author: MarcoFalke

configure.ac

@@ -323,6 +323,7 @@ if test "x$enable_werror" = "xyes"; then
     AC_MSG_ERROR("enable-werror set but -Werror is not usable")
   fi
   AX_CHECK_COMPILE_FLAG([-Werror=vla],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=vla"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Werror=switch],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=switch"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=thread-safety-analysis],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=thread-safety-analysis"],,[[$CXXFLAG_WERROR]])
 fi
 
@@ -331,6 +332,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wextra],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wextra"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wformat],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wformat"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wvla],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wvla"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wswitch],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wswitch"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wformat-security],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wformat-security"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wthread-safety-analysis],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wthread-safety-analysis"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wrange-loop-analysis],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wrange-loop-analysis"],,[[$CXXFLAG_WERROR]])