Makefile: Track changes to LDFLAGS and relink when necessary

Some profiling tools (e.g., google-perftools and mutrace) work by
linking in a new library into the executables. When using these tools
it is convenient to only relink instead of doing a full make clean;
make cycle.

This change complements the auto-detection of changes to CFLAGS that
we already have. Tracking of more variables that affect the build can
be added when the need arise.

Signed-off-by: Fredrik Kuivinen <frekui@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -1,5 +1,6 @@
 /GIT-BUILD-OPTIONS
 /GIT-CFLAGS
+/GIT-LDFLAGS
 /GIT-GUI-VARS
 /GIT-VERSION-FILE
 /bin-wrappers/

Makefile

@@ -1706,7 +1706,7 @@ git.sp git.s git.o: EXTRA_CPPFLAGS = -DGIT_VERSION='"$(GIT_VERSION)"' \
 	'-DGIT_MAN_PATH="$(mandir_SQ)"' \
 	'-DGIT_INFO_PATH="$(infodir_SQ)"'
 
-git$X: git.o $(BUILTIN_OBJS) $(GITLIBS)
+git$X: git.o GIT-LDFLAGS $(BUILTIN_OBJS) $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ git.o \
 		$(BUILTIN_OBJS) $(ALL_LDFLAGS) $(LIBS)
 
@@ -2004,17 +2004,17 @@ compat/nedmalloc/nedmalloc.sp compat/nedmalloc/nedmalloc.o: EXTRA_CPPFLAGS = \
 	-DNDEBUG -DOVERRIDE_STRDUP -DREPLACE_SYSTEM_ALLOCATOR
 endif
 
-git-%$X: %.o $(GITLIBS)
+git-%$X: %.o GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(LIBS)
 
-git-imap-send$X: imap-send.o $(GITLIBS)
+git-imap-send$X: imap-send.o GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIBS) $(OPENSSL_LINK) $(OPENSSL_LIBSSL) $(LIB_4_CRYPTO)
 
-git-http-fetch$X: revision.o http.o http-walker.o http-fetch.o $(GITLIBS)
+git-http-fetch$X: revision.o http.o http-walker.o http-fetch.o GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIBS) $(CURL_LIBCURL)
-git-http-push$X: revision.o http.o http-push.o $(GITLIBS)
+git-http-push$X: revision.o http.o http-push.o GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIBS) $(CURL_LIBCURL) $(EXPAT_LIBEXPAT)
 
@@ -2024,7 +2024,7 @@ $(REMOTE_CURL_ALIASES): $(REMOTE_CURL_PRIMARY)
 	ln -s $< $@ 2>/dev/null || \
 	cp $< $@
 
-$(REMOTE_CURL_PRIMARY): remote-curl.o http.o http-walker.o $(GITLIBS)
+$(REMOTE_CURL_PRIMARY): remote-curl.o http.o http-walker.o GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIBS) $(CURL_LIBCURL) $(EXPAT_LIBEXPAT)
 
@@ -2094,6 +2094,15 @@ GIT-CFLAGS: FORCE
 		echo "$$FLAGS" >GIT-CFLAGS; \
             fi
 
+TRACK_LDFLAGS = $(subst ','\'',$(ALL_LDFLAGS))
+
+GIT-LDFLAGS: FORCE
+	@FLAGS='$(TRACK_LDFLAGS)'; \
+	    if test x"$$FLAGS" != x"`cat GIT-LDFLAGS 2>/dev/null`" ; then \
+		echo 1>&2 "    * new link flags"; \
+		echo "$$FLAGS" >GIT-LDFLAGS; \
+            fi
+
 # We need to apply sq twice, once to protect from the shell
 # that runs GIT-BUILD-OPTIONS, and then again to protect it
 # and the first level quoting from the shell that runs "echo".
@@ -2165,7 +2174,7 @@ test-svn-fe$X: vcs-svn/lib.a
 
 .PRECIOUS: $(TEST_OBJS)
 
-test-%$X: test-%.o $(GITLIBS)
+test-%$X: test-%.o GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(filter %.a,$^) $(LIBS)
 
 check-sha1:: test-sha1$X
@@ -2375,7 +2384,7 @@ ifndef NO_TCLTK
 	$(MAKE) -C gitk-git clean
 	$(MAKE) -C git-gui clean
 endif
-	$(RM) GIT-VERSION-FILE GIT-CFLAGS GIT-GUI-VARS GIT-BUILD-OPTIONS
+	$(RM) GIT-VERSION-FILE GIT-CFLAGS GIT-LDFLAGS GIT-GUI-VARS GIT-BUILD-OPTIONS
 
 .PHONY: all install clean strip
 .PHONY: shell_compatibility_test please_set_SHELL_PATH_to_a_more_modern_shell