Silence GCC warnings when using libc++ headers.

GCC 12 raises warnings about literal operator suffixes not preceded by
'_' in libc++ headers such as <string_view> as it doesn't recognize
libc++ headers being an implementation of the standard.

GCC 12 also warns about clang-specific pragmas in <locale>.

Disabling these warnings globally for all C++ code is not ideal, but
is a better option than patching libc++ headers to ignore these
warnings.

Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D37530

share/mk/bsd.sys.mk

@@ -217,6 +217,14 @@ CWARNFLAGS+=	-Wno-error=overflow
 .endif
 .endif
 
+# GCC 12.1.0
+.if ${COMPILER_VERSION} >= 120100
+# These warnings are raised by headers in libc++ so are disabled
+# globally for all C++
+CXXWARNFLAGS+=	-Wno-literal-suffix 			\
+		-Wno-error=unknown-pragmas
+.endif
+
 # GCC produces false positives for functions that switch on an
 # enum (GCC bug 87950)
 CWARNFLAGS+=	-Wno-return-type