Makefile: whitespace style fixes in macro definitions

Consistently use a single space before and after the "=" (or ":=", "+=",
etc.) in assignments to make macros.  Granted, this was not a big deal,
but I did find the needless inconsistency quite distracting.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/config.mak.in b/config.mak.in
index 69d4838..e8a9bb4 100644
--- a/config.mak.in
+++ b/config.mak.in
@@ -18,7 +18,7 @@
 template_dir = @datadir@/git-core/templates
 sysconfdir = @sysconfdir@
 
-mandir=@mandir@
+mandir = @mandir@
 
 srcdir = @srcdir@
 VPATH = @srcdir@