Fix compilation with Sun CC

Fix compilation with Sun CC

- Add the CFLAGS variable to config.mak.in to override the Makefile's
  default, which is gcc-specific and won't work with Sun CC.
- Prefer "cc" over "gcc", because Pasky's Git.pm will not compile with gcc
  on Solaris at all. On Linux and the free BSDs "cc" is linked to "gcc"
  anyway.
- Set correct flag to generate position-independent code.
- Add "-xO3" (= use default optimization level) to CFLAGS.

Signed-off-by: Dennis Stosberg <dennis@stosberg.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>