legacysupport PG trips up GCC C compiler with -nostdinc++

[RJVB (René Bertin)]

Trying to build port:vte with macports-gcc-12, I get:

meson.build:142:0: ERROR: Assert failed: option -std=gnu11 not supported by gcc

A full log can be found at ${build.dir}/meson-logs/meson-log.txt

and that log file shows that the C compiler is invoked (to determine if it supports the gnu11 C dialect) with -nostdinc++.

That argument is inserted by the legacysupport-1.1 PG into the CPPFLAGS when use_mp_libcxx is set. That is an anomaly which happens to be tolerated by the clang C compiler but not by GCC, which rightfully points out that the argument is valid only for C++ and ObjC++.

Adding an additional ls_cache_cxxflags variable to the PG which inserts into configure.cxxflags fixes the issue.