Guard a few Makefile variables against user environments

Guard a few Makefile variables against user environments

Some variables are not initialized in the Makefile, but appended to.  If
the user has those variables in her environment, it will break the
build.

The variable names were found using these commands:

	$ s='[ \t]';
	$ S='[^ \t]';
	$ comm -23 \
		<(sed -n "s/^$s*\($S*\)$s$s*+=.*/\1/p" < Makefile |
		  sort | uniq) \
		<(sed -n "s/^$s*\($S*\)$s$s*=.*/\1/p" < Makefile |
		  sort | uniq)

This fixes msysGit issue 216.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>