Fixes several problems:
* include config.mak.uname, config.mak.autogen and config.mak
in order to use settings for prefix and other such things;
* link xdiff/lib.a as it is a requirement for libgit.a;
* fix CFLAGS, LDFLAGS and EXTLIBS for Linux and Mac OS X.
Signed-off-by: Maxim Bublis <email@example.com>
Signed-off-by: Junio C Hamano <firstname.lastname@example.org>