D: strip unsupported -D (and -f) flags for ldc2

[valpackett]

Building tilix on FreeBSD, where the x11 pkgconf file contains a define flag:

% cat /usr/local/libdata/pkgconfig/x11.pc
[…]
Cflags: -I${includedir} -D_THREAD_SAFE

[1/63] ldc2 -I=tilix@exe -I=. -I=.. -I=../source/ -I=data -I/usr/local/include/d/gtkd-3/ -I/usr/local/include -enable-color -wi -release -enable-inlining -Hkeep-all-bodies -O3 -D_THREAD_SAFE -vcolumns -d-version=StdLoggerDisableTrace  -of='tilix@exe/source_secretc_secret.d.o' -c ../source/secretc/secret.d
FAILED: tilix@exe/source_secretc_secret.d.o
ldc2 -I=tilix@exe -I=. -I=.. -I=../source/ -I=data -I/usr/local/include/d/gtkd-3/ -I/usr/local/include -enable-color -wi -release -enable-inlining -Hkeep-all-bodies -O3 -D_THREAD_SAFE -vcolumns -d-version=StdLoggerDisableTrace  -of='tilix@exe/source_secretc_secret.d.o' -c ../source/secretc/secret.d
ldc2: Unknown command line argument '-D_THREAD_SAFE'.  Try: 'ldc2 -help'
ldc2: Did you mean '-pre-RA-sched'?

I guess such flags should be just removed…

Also, FreeBSD Ports sets up LDFLAGS for C/C++, and meson ends up passing unsupported flags to ldc2 here as well:

ldc2: Unknown command line argument '-fstack-protector-strong'.  Try: 'ldc2 -help'
ldc2: Did you mean '-stack-symbol-ordering'?

[valpackett]

ping