Add NO_CROSS_DIRECTORY_HARDLINKS support to the Makefile

When the installed programs are tar'ed up and installed on a system where
bin/ and libexec/git-core/ live on different file systems, we do not want
libexec/git-core/git-* to be hardlinks to bin/git.

Noticed by Cedric Staniewski.

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

Makefile

@@ -159,6 +159,9 @@ all::
 # Define NO_EXTERNAL_GREP if you don't want "git grep" to ever call
 # your external grep (e.g., if your system lacks grep, if its grep is
 # broken, or spawning external process is slower than built-in grep git has).
+#
+# Define NO_CROSS_DIRECTORY_HARDLINKS if you plan to distribute the installed
+# programs as a tar, where bin/ and libexec/ might be on different file systems.
 
 GIT-VERSION-FILE: .FORCE-GIT-VERSION-FILE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -1468,6 +1471,7 @@ endif
 	bindir=$$(cd '$(DESTDIR_SQ)$(bindir_SQ)' && pwd) && \
 	execdir=$$(cd '$(DESTDIR_SQ)$(gitexec_instdir_SQ)' && pwd) && \
 	{ $(RM) "$$execdir/git-add$X" && \
+		test -z "$(NO_CROSS_DIRECTORY_HARDLINKS)" && \
 		ln "$$bindir/git$X" "$$execdir/git-add$X" 2>/dev/null || \
 		cp "$$bindir/git$X" "$$execdir/git-add$X"; } && \
 	{ for p in $(filter-out git-add$X,$(BUILT_INS)); do \