Log Message:
python3[4567]: find_library() fixes; remove -stack_size from LDFLAGS on Darwin.

PkgSrc changes:
* Make find_library() consitant for all Python versions:
  - Fallback to clang, when gcc is not installed.
  - Find libraries in PkgSrc prefix.
* Do not use -stack_size; it ends up in 'pythonNN-config --ldflags', and some
  modules fail to build.