Use targeted env vars for macOS build instructions #1057

Pr. now, we unfortunately encourage setting CFLAGS and LDFLAGS for the scope of the entire build, simply to satisfy the build requirements of the _gdbm and _lzma extension modules.

Following the recent build system improvements, we've now got targeted variables for this, such as GDBM_CFLAGS and GDBM_LIBS. We should use them.

This is a follow-up to #1052 (and various other recent macOS build instruction improvements).