Makefile: split prefix flags from GIT-CFLAGS

Makefile: split prefix flags from GIT-CFLAGS

Most of the build targets do not care about the setting of
$prefix (or its derivative variables), but will be rebuilt
if the prefix changes. For most setups this doesn't matter
(they set prefix once and never change it), but for a setup
which puts each branch or version in its own prefix, this
unnecessarily causes a full rebuild whenever the branc is
changed.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>