Nothing interesting in this commit, just moving files around, making
sure every target compiles every file, and controlling with macros
instead.
Basic support for fxg3a target in CMakeLists.txt, cmake/FindGint.cmake,
giteapc.make and include/gint/config.h.in. The rest is forgettable.
Known limitation: OpenLibm can't be installed out of the compiler prefix
yet (because of that pesky openlibm/ prefix that it installs to but does
not use when including).
