lang/gcc12-devel: Expose non-default -stdlib=libc++ support

Fix -stdlib=libc++ option which produced "error: unrecognized
command-line option '-stdlib=libc++'".

PR:		265962
Reported by:	jbeich

lang/gcc12-devel/Makefile

@@ -104,6 +104,7 @@ CONFIGURE_ARGS+=--disable-nls \
 		--with-as=${LOCALBASE}/bin/as \
 		--with-gmp=${LOCALBASE} \
 		--with-gxx-include-dir=${TARGLIB}/include/c++/ \
+		--with-gxx-libcxx-include-dir=/usr/include/c++/v1 \
 		--with-ld=${LOCALBASE}/bin/ld \
 		${ICONV_CONFIGURE_ARG} \
 		--with-pkgversion="FreeBSD Ports Collection" \

lang/gcc12-devel/files/patch-libcxxrt

+libc++ on FreeBSD always uses PathScale libcxxrt and cannot change to
+LLVM libc++abi without breaking backward compatibility. Besides, mixing
+different C++ ABIs is not supported unless subset via DT_FILTER.
+
+https://github.com/llvm/llvm-project/commit/35479ffb1251
+https://github.com/freebsd/freebsd-src/commit/cf56074e5271
+
+--- gcc/cp/g++spec.cc.orig	2022-08-06 22:32:19 UTC
++++ gcc/cp/g++spec.cc
+@@ -60,7 +60,7 @@ along with GCC; see the file COPYING3.  If not see
+ #endif
+ 
+ #ifndef LIBCXXABI
+-#define LIBCXXABI "c++abi"
++#define LIBCXXABI "cxxrt"
+ #endif
+ #ifndef LIBCXXABI_PROFILE
+ #define LIBCXXABI_PROFILE LIBCXXABI