meson: Fix build failure with gcc10 on mingw
requested to merge nirbheek/fontconfig:pickup-cflags-ldflags-from-env-vars-when-cross-compiling into master
We were explicitly setting c_args and c_link_args to [], which overrode the values inherited from the env via CFLAGS and LDFLAGS.
Also add a comment for future reference.
Fixes #263 (closed)