gitweb/Makefile: prepare to merge into top-level Makefile

gitweb/Makefile: prepare to merge into top-level Makefile

Since the "gitweb/Makefile" was split out from the top-level Makefile
in 62331ef1637 (gitweb: Makefile improvements, 2010-01-30) we've kept
the inter-dependencies between the two, and worse have dealt with a
lot of duplication as a result.

In preparation for merging the two again add a MAK_DIR_GITWEB variable
to various rules in it. This will allow us to set this variable to
"gitweb/" as we include it in the top-level Makefile, which will
minimize the size of the subsequent diff.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>