This task originated from this comment in the code review for PHPC-759.

There are more pkg-config options than I realized. Per pkg-config(1), we can request -I CFLAGS and "other" CFLAGS independently, which would let us rely on PHP_EVAL_INCLINE and defer to PHP_MONGODB_CFLAGS when necessary. Similarly, we can requires -L and -l libs independently from "other" linker arguments (which might require manual appends instead of PHP_EVAL_LIBLINE). While this is trivial to add for libbson and libmongoc, since we call pkg-config directly, we also know by looking at their package files that it would have no functional impact. It could be helpful for other libraries (e.g. CheckSSL.m4), but those are all using the pkg.m4 macro at the moment and would require some significant refactoring. I don't think there's a pressing need for this, but it does seem like it'd be the "correct" approach.