Use targeted env vars for macOS build instructions

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).