Makefile: detect when PYTHON_PATH changes

When make is run, the python scripts are created from *.py files that
are changed to use the python given by PYTHON_PATH. And PYTHON_PATH
is set by default to /usr/bin/python on Linux.

However, next time make is run with a different value in PYTHON_PATH,
we failed to regenerate these scripts.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -3,6 +3,7 @@
 /GIT-LDFLAGS
 /GIT-GUI-VARS
 /GIT-PREFIX
+/GIT-PYTHON-VARS
 /GIT-SCRIPT-DEFINES
 /GIT-USER-AGENT
 /GIT-VERSION-FILE

Makefile

@@ -2245,7 +2245,7 @@ $(patsubst %.perl,%,$(SCRIPT_PERL)) git-instaweb: % : unimplemented.sh
 endif # NO_PERL
 
 ifndef NO_PYTHON
-$(patsubst %.py,%,$(SCRIPT_PYTHON)): GIT-CFLAGS GIT-PREFIX
+$(patsubst %.py,%,$(SCRIPT_PYTHON)): GIT-CFLAGS GIT-PREFIX GIT-PYTHON-VARS
 $(patsubst %.py,%,$(SCRIPT_PYTHON)): % : %.py
 	$(QUIET_GEN)$(RM) $@ $@+ && \
 	INSTLIBDIR=`MAKEFLAGS= $(MAKE) -C git_remote_helpers -s \
@@ -2636,6 +2636,18 @@ GIT-GUI-VARS: FORCE
             fi
 endif
 
+### Detect Python interpreter path changes
+ifndef NO_PYTHON
+TRACK_PYTHON = $(subst ','\'',-DPYTHON_PATH='$(PYTHON_PATH_SQ)')
+
+GIT-PYTHON-VARS: FORCE
+	@VARS='$(TRACK_PYTHON)'; \
+	    if test x"$$VARS" != x"`cat $@ 2>/dev/null`" ; then \
+		echo 1>&2 "    * new Python interpreter location"; \
+		echo "$$VARS" >$@; \
+            fi
+endif
+
 test_bindir_programs := $(patsubst %,bin-wrappers/%,$(BINDIR_PROGRAMS_NEED_X) $(BINDIR_PROGRAMS_NO_X) $(TEST_PROGRAMS_NEED_X))
 
 all:: $(TEST_PROGRAMS) $(test_bindir_programs)
@@ -2911,7 +2923,7 @@ ifndef NO_TCLTK
 	$(MAKE) -C git-gui clean
 endif
 	$(RM) GIT-VERSION-FILE GIT-CFLAGS GIT-LDFLAGS GIT-GUI-VARS GIT-BUILD-OPTIONS
-	$(RM) GIT-USER-AGENT GIT-PREFIX GIT-SCRIPT-DEFINES
+	$(RM) GIT-USER-AGENT GIT-PREFIX GIT-SCRIPT-DEFINES GIT-PYTHON-VARS
 
 .PHONY: all install profile-clean clean strip
 .PHONY: shell_compatibility_test please_set_SHELL_PATH_to_a_more_modern_shell