Some not-too-smallish changes to configure.ac — the end result is a visibly faster run of ./configure.
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.