CONFIG: add Gcc rules for MacOS (darwin)

- /usr/bin/{gcc,g++} normally just symlinks to clang/clang++
  and may have unknown default flags.
  For a gcc toolchain, it would be better to use a homebrew
  installation.

  For these cases, the compiler will need to be specified with
  version=.. in WM_COMPILE_CONTROL.

  For example, with "version=14", to select gcc-14, g++-14 from the
  homebrew installation.

- needs a slight hack for locating the FlexLexer.h header.
  Added into src/OSspecific/POSIX similar to how it is handled
  in src/OSspecific/MSwindows

CONFIG: add simple config/detection support for libumpire (Linux)