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 <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -2,6 +2,7 @@
 /GIT-CFLAGS
 /GIT-LDFLAGS
 /GIT-GUI-VARS
+/GIT-PREFIX
 /GIT-USER-AGENT
 /GIT-VERSION-FILE
 /bin-wrappers/

Makefile

@@ -1973,6 +1973,7 @@ shell_compatibility_test: please_set_SHELL_PATH_to_a_more_modern_shell
 strip: $(PROGRAMS) git$X
 	$(STRIP) $(STRIP_OPTS) $(PROGRAMS) git$X
 
+git.sp git.s git.o: GIT-PREFIX
 git.sp git.s git.o: EXTRA_CPPFLAGS = \
 	'-DGIT_HTML_PATH="$(htmldir_SQ)"' \
 	'-DGIT_MAN_PATH="$(mandir_SQ)"' \
@@ -1984,7 +1985,7 @@ git$X: git.o GIT-LDFLAGS $(BUILTIN_OBJS) $(GITLIBS)
 
 help.sp help.s help.o: common-cmds.h
 
-builtin/help.sp builtin/help.s builtin/help.o: common-cmds.h
+builtin/help.sp builtin/help.s builtin/help.o: common-cmds.h GIT-PREFIX
 builtin/help.sp builtin/help.s builtin/help.o: EXTRA_CPPFLAGS = \
 	'-DGIT_HTML_PATH="$(htmldir_SQ)"' \
 	'-DGIT_MAN_PATH="$(mandir_SQ)"' \
@@ -2031,7 +2032,7 @@ $(SCRIPT_LIB) : % : %.sh
 ifndef NO_PERL
 $(patsubst %.perl,%,$(SCRIPT_PERL)): perl/perl.mak
 
-perl/perl.mak: GIT-CFLAGS perl/Makefile perl/Makefile.PL
+perl/perl.mak: GIT-CFLAGS GIT-PREFIX perl/Makefile perl/Makefile.PL
 	$(QUIET_SUBDIR0)perl $(QUIET_SUBDIR1) PERL_PATH='$(PERL_PATH_SQ)' prefix='$(prefix_SQ)' $(@F)
 
 $(patsubst %.perl,%,$(SCRIPT_PERL)): % : %.perl
@@ -2075,7 +2076,7 @@ $(patsubst %.perl,%,$(SCRIPT_PERL)) git-instaweb: % : unimplemented.sh
 endif # NO_PERL
 
 ifndef NO_PYTHON
-$(patsubst %.py,%,$(SCRIPT_PYTHON)): GIT-CFLAGS
+$(patsubst %.py,%,$(SCRIPT_PYTHON)): GIT-CFLAGS GIT-PREFIX
 $(patsubst %.py,%,$(SCRIPT_PYTHON)): % : %.py
 	$(QUIET_GEN)$(RM) $@ $@+ && \
 	INSTLIBDIR=`MAKEFLAGS= $(MAKE) -C git_remote_helpers -s \
@@ -2235,20 +2236,25 @@ else
 $(OBJECTS): $(LIB_H)
 endif
 
+exec_cmd.sp exec_cmd.s exec_cmd.o: GIT-PREFIX
 exec_cmd.sp exec_cmd.s exec_cmd.o: EXTRA_CPPFLAGS = \
 	'-DGIT_EXEC_PATH="$(gitexecdir_SQ)"' \
 	'-DBINDIR="$(bindir_relative_SQ)"' \
 	'-DPREFIX="$(prefix_SQ)"'
 
+builtin/init-db.sp builtin/init-db.s builtin/init-db.o: GIT-PREFIX
 builtin/init-db.sp builtin/init-db.s builtin/init-db.o: EXTRA_CPPFLAGS = \
 	-DDEFAULT_GIT_TEMPLATE_DIR='"$(template_dir_SQ)"'
 
+config.sp config.s config.o: GIT-PREFIX
 config.sp config.s config.o: EXTRA_CPPFLAGS = \
 	-DETC_GITCONFIG='"$(ETC_GITCONFIG_SQ)"'
 
+attr.sp attr.s attr.o: GIT-PREFIX
 attr.sp attr.s attr.o: EXTRA_CPPFLAGS = \
 	-DETC_GITATTRIBUTES='"$(ETC_GITATTRIBUTES_SQ)"'
 
+gettext.sp gettext.s gettext.o: GIT-PREFIX
 gettext.sp gettext.s gettext.o: EXTRA_CPPFLAGS = \
 	-DGIT_LOCALE_PATH='"$(localedir_SQ)"'
 
@@ -2372,14 +2378,22 @@ cscope:
 	$(FIND_SOURCE_FILES) | xargs cscope -b
 
 ### Detect prefix changes
-TRACK_CFLAGS = $(CC):$(subst ','\'',$(ALL_CFLAGS)):\
-             $(bindir_SQ):$(gitexecdir_SQ):$(template_dir_SQ):$(prefix_SQ):\
-             $(localedir_SQ):$(USE_GETTEXT_SCHEME)
+TRACK_PREFIX = $(bindir_SQ):$(gitexecdir_SQ):$(template_dir_SQ):$(prefix_SQ):\
+		$(localedir_SQ)
+
+GIT-PREFIX: FORCE
+	@FLAGS='$(TRACK_PREFIX)'; \
+	if test x"$$FLAGS" != x"`cat GIT-PREFIX 2>/dev/null`" ; then \
+		echo 1>&2 "    * new prefix flags"; \
+		echo "$$FLAGS" >GIT-PREFIX; \
+	fi
+
+TRACK_CFLAGS = $(CC):$(subst ','\'',$(ALL_CFLAGS)):$(USE_GETTEXT_SCHEME)
 
 GIT-CFLAGS: FORCE
 	@FLAGS='$(TRACK_CFLAGS)'; \
 	    if test x"$$FLAGS" != x"`cat GIT-CFLAGS 2>/dev/null`" ; then \
-		echo 1>&2 "    * new build flags or prefix"; \
+		echo 1>&2 "    * new build flags"; \
 		echo "$$FLAGS" >GIT-CFLAGS; \
             fi
 
@@ -2715,7 +2729,7 @@ 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
+	$(RM) GIT-USER-AGENT GIT-PREFIX
 
 .PHONY: all install profile-clean clean strip
 .PHONY: shell_compatibility_test please_set_SHELL_PATH_to_a_more_modern_shell