Makefile: include Git's config.mak.uname

This pulls in the correct value of $(INSTALL) on a wide variety of
systems.

Signed-off-by: John Keeping <john@keeping.me.uk>

Makefile

@@ -33,6 +33,7 @@ DOC_PDF  = $(patsubst %.txt,%.pdf,$(MAN_TXT))
 
 #-include config.mak
 
+include git/config.mak.uname
 #
 # Let the user override the above settings.
 #