Implicitly include config.h in all files

Implicitly include config.h in all files

The definitions in config.h should be available in all files an
implementation detail; it can be included through the build system
instead of having each file pull it every time.

This is especially helpful with AC_USE_SYSTEM_EXTENSIONS, as _GNU_SOURCE
and friends can have an effect by merely being defined, which can lead
to some confusion if its effective for only half the files.

And we don't really support a build _without_ config.h; so, one less
thing to worry about.

Signed-off-by: Ran Benita <ran234@gmail.com>