Makefile: improve dependency generation

Makefile: improve dependency generation

Makefile target generation would always be included for any makefile
target that was not clean.  Only care to include the '.deps' directory
when building cgit, rather than generating and including dependencies
when calling other makefile targets.

Heavily borrowed from git's Makefile, but without definitions to test
for the compiler's header dependency feature.  Previous Makefile
implementation never checked for this compiler feature anyway.

  - Removed makecmdgoal 'clean' check
  - Grouped like .PHONY target definitions
  - Place build dependency targets under .SUFFIXES
  - Re-arranged location of library inclusion definitions
  - Use google code mirror instead of github

Signed-off-by: Jamie Couture <jamie.couture@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>