auto-detect changed prefix and/or changed build flags

Detect changed prefix and/or changed build flags in the middle
of the build (or between 'make' and 'make install'), and if change
is detected, make sure all objects are compiled with same build
flags and same prefix, thus avoiding inconsistent/broken build.

[jc: removed otherwise unnecessary Makefile target to test the
 change this patch introduces. ]

Signed-off-by: Yakov Lerner <iler.ml@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -464,6 +464,7 @@ DESTDIR_SQ = $(subst ','\'',$(DESTDIR))
 bindir_SQ = $(subst ','\'',$(bindir))
 gitexecdir_SQ = $(subst ','\'',$(gitexecdir))
 template_dir_SQ = $(subst ','\'',$(template_dir))
+prefix_SQ = $(subst ','\'',$(prefix))
 
 SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))
 PERL_PATH_SQ = $(subst ','\'',$(PERL_PATH))
@@ -484,7 +485,7 @@ all:
 strip: $(PROGRAMS) git$X
 	$(STRIP) $(STRIP_OPTS) $(PROGRAMS) git$X
 
-git$X: git.c common-cmds.h $(BUILTIN_OBJS) $(GITLIBS)
+git$X: git.c common-cmds.h $(BUILTIN_OBJS) $(GITLIBS) GIT-CFLAGS
 	$(CC) -DGIT_VERSION='"$(GIT_VERSION)"' \
 		$(ALL_CFLAGS) -o $@ $(filter %.c,$^) \
 		$(BUILTIN_OBJS) $(ALL_LDFLAGS) $(LIBS)
@@ -516,7 +517,7 @@ $(patsubst %.perl,%,$(SCRIPT_PERL)) : % : %.perl
 	chmod +x $@+
 	mv $@+ $@
 
-$(patsubst %.py,%,$(SCRIPT_PYTHON)) : % : %.py
+$(patsubst %.py,%,$(SCRIPT_PYTHON)) : % : %.py GIT-CFLAGS
 	rm -f $@ $@+
 	sed -e '1s|#!.*python|#!$(PYTHON_PATH_SQ)|' \
 	    -e 's|@@GIT_PYTHON_PATH@@|$(GIT_PYTHON_DIR_SQ)|g' \
@@ -540,19 +541,19 @@ git$X git.spec \
 	$(patsubst %.py,%,$(SCRIPT_PYTHON)) \
 	: GIT-VERSION-FILE
 
-%.o: %.c
+%.o: %.c GIT-CFLAGS
 	$(CC) -o $*.o -c $(ALL_CFLAGS) $<
 %.o: %.S
 	$(CC) -o $*.o -c $(ALL_CFLAGS) $<
 
-exec_cmd.o: exec_cmd.c
+exec_cmd.o: exec_cmd.c GIT-CFLAGS
 	$(CC) -o $*.o -c $(ALL_CFLAGS) '-DGIT_EXEC_PATH="$(gitexecdir_SQ)"' $<
 
-http.o: http.c
+http.o: http.c GIT-CFLAGS
 	$(CC) -o $*.o -c $(ALL_CFLAGS) -DGIT_USER_AGENT='"git/$(GIT_VERSION)"' $<
 
 ifdef NO_EXPAT
-http-fetch.o: http-fetch.c http.h
+http-fetch.o: http-fetch.c http.h GIT-CFLAGS
 	$(CC) -o $*.o -c $(ALL_CFLAGS) -DNO_EXPAT $<
 endif
 
@@ -609,6 +610,17 @@ tags:
 	rm -f tags
 	find . -name '*.[hcS]' -print | xargs ctags -a
 
+### Detect prefix changes
+TRACK_CFLAGS = $(subst ','\'',$(ALL_CFLAGS)):$(GIT_VERSION):\
+             $(bindir_SQ):$(gitexecdir_SQ):$(template_dir_SQ):$(prefix_SQ)
+
+GIT-CFLAGS: .FORCE-GIT-CFLAGS
+	@FLAGS='$(TRACK_CFLAGS)'; \
+	    if test x"$$FLAGS" != x"`cat GIT-CFLAGS 2>/dev/null`" ; then \
+		echo 1>&2 "    * new build flags or prefix"; \
+		echo "$$FLAGS" >GIT-CFLAGS; \
+            fi
+
 ### Testing rules
 
 # GNU make supports exporting all variables by "export" without parameters.
@@ -711,10 +723,10 @@ clean:
 	$(MAKE) -C Documentation/ clean
 	$(MAKE) -C templates clean
 	$(MAKE) -C t/ clean
-	rm -f GIT-VERSION-FILE
+	rm -f GIT-VERSION-FILE GIT-CFLAGS
 
 .PHONY: all install clean strip
-.PHONY: .FORCE-GIT-VERSION-FILE TAGS tags
+.PHONY: .FORCE-GIT-VERSION-FILE TAGS tags .FORCE-GIT-CFLAGS
 
 ### Check documentation
 #