Merge #18882: build: fix -Wformat-security check when compiling with GCC

6cef365 build: fix -Wformat-security check when compiling with GCC (fanquake)

Pull request description:

  GCC expects `-Wformat` to be passed with [`-Wformat-security`](https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html), which means
  when we test for it in configure it currently fails:

  ```bash
  checking whether C++ compiler accepts -Wformat-security... no
  ...
  configure:15907: checking whether C++ compiler accepts -Wformat-security
  configure:15926: g++ -std=c++11 -c -g -O2 -Werror -Wformat-security  conftest.cpp >&5
  cc1plus: error: '-Wformat-security' ignored without '-Wformat' [-Werror=format-security]
  cc1plus: all warnings being treated as errors
  ```

  and never gets added to our CXX flags. Note that Clang does not have this requirement and the check is working correctly there.

  The change in this PR is the simple fix, however we might want to consider using something like `-Wformat=2` in future, which in GCC is equivalent to `-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.` and similar [in Clang](https://clang.llvm.org/docs/DiagnosticsReference.html#wformat-2).

ACKs for top commit:
  practicalswift:
    ACK 6cef365
  laanwj:
    ACK 6cef365

Tree-SHA512: f9230d42af39f85ea9d2f55dbbebd2bae4740fe59b0da2e092af3ac9ef7e6799d3a4cf83eb64574c63982e5f6b14e226d44c84fa0335255d65c9947d86a1ea38

Author: laanwj

configure.ac

@@ -392,11 +392,11 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wall],[WARN_CXXFLAGS="$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"],,[[$CXXFLAG_WERROR]])
-  AX_CHECK_COMPILE_FLAG([-Wformat],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wformat"],,[[$CXXFLAG_WERROR]])
+  dnl some compilers will ignore -Wformat-security without -Wformat, so just combine the two here.
+  AX_CHECK_COMPILE_FLAG([-Wformat -Wformat-security],[WARN_CXXFLAGS="$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"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wswitch],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wswitch"],,[[$CXXFLAG_WERROR]])
-  AX_CHECK_COMPILE_FLAG([-Wformat-security],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wformat-security"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wthread-safety],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wthread-safety"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wrange-loop-analysis],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wrange-loop-analysis"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wredundant-decls],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wredundant-decls"],,[[$CXXFLAG_WERROR]])