Makefile maintenance

Makefile maintenance

Triggered by fixing compilation in MacOS.

- Rename extension.h to ext.h; the former collides with Extension.h.
- Move _DEFAULT_SOURCE to the source; it's not widespread enough for
  Makefile.am.
- Add _DARWIN_C_SOURCE, needed by MacOS for timegm() and mkdtemp().
- Add -flto to unit test AM_CFLAGS. This minimizes superflous #includes
  and mocks needed, and will hopefully make them consistent across
  platforms.
- Delete _BSD_SOURCE; it seems orphaned. (Though see below.)

Works on Linux and Mac. Might have broken the BSDs; I can't test them
ATM.