build: fix out-of-tree build/installs.

When doing an out-of-tree install, gource.1 is not on the work
directory so make sure to use $(srcdir) there.
build: use pkg-config to find dependencies.

pkg-config is friendlier to cross-compilation, and all these libraries
nowadays use it natively.

This also allows to drop the m4 files for freetype and sdl.
build: use a single call to AC_CHECK_FUNCS.

This simplifies the configure with no difference in final output.
Merge pull request #20 from Flameeyes/autotools-improvements

Autotools improvements