fundamental-macro: conditionalize several gcc warning pragmas

fundamental-macro: conditionalize several gcc warning pragmas

This fixes the following error:

In file included from ../src/basic/macro.h:13,
                 from ../src/basic/env-util.h:9,
                 from ../src/nss-systemd/nss-systemd.c:7:
../src/fundamental/macro-fundamental.h:61:9: error: option ‘-Wzero-as-null-pointer-constant’ is valid for C++/ObjC++ but not for C [-Werror=pragmas]
   61 |         _Pragma("GCC diagnostic ignored \"-Wzero-as-null-pointer-constant\"")
      |         ^~~~~~~
../src/nss-systemd/nss-systemd.c:106:1: note: in expansion of macro ‘DISABLE_WARNING_ZERO_AS_NULL_POINTER_CONSTANT’
  106 | DISABLE_WARNING_ZERO_AS_NULL_POINTER_CONSTANT;
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors