Makefile: split GIT_USER_AGENT from GIT-CFLAGS

The default user-agent depends on the GIT_VERSION, which
means that anytime you switch versions, it causes a full
rebuild. Instead, let's split it out into its own file and
restrict the dependency to version.o.

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-USER-AGENT
 /GIT-VERSION-FILE
 /bin-wrappers/
 /git

Makefile

@@ -1922,7 +1922,11 @@ endif
 GIT_USER_AGENT_SQ = $(subst ','\'',$(GIT_USER_AGENT))
 GIT_USER_AGENT_CQ = "$(subst ",\",$(subst \,\\,$(GIT_USER_AGENT)))"
 GIT_USER_AGENT_CQ_SQ = $(subst ','\'',$(GIT_USER_AGENT_CQ))
-BASIC_CFLAGS += -DGIT_USER_AGENT='$(GIT_USER_AGENT_CQ_SQ)'
+GIT-USER-AGENT: FORCE
+	@if test x'$(GIT_USER_AGENT_SQ)' != x"`cat GIT-USER-AGENT 2>/dev/null`"; then \
+		echo >&2 "    * new user-agent flag"; \
+		echo '$(GIT_USER_AGENT_SQ)' >GIT-USER-AGENT; \
+	fi
 
 ALL_CFLAGS += $(BASIC_CFLAGS)
 ALL_LDFLAGS += $(BASIC_LDFLAGS)
@@ -1987,8 +1991,10 @@ builtin/help.sp builtin/help.s builtin/help.o: EXTRA_CPPFLAGS = \
 	'-DGIT_MAN_PATH="$(mandir_SQ)"' \
 	'-DGIT_INFO_PATH="$(infodir_SQ)"'
 
+version.sp version.s version.o: GIT-USER-AGENT
 version.sp version.s version.o: EXTRA_CPPFLAGS = \
-	'-DGIT_VERSION="$(GIT_VERSION)"'
+	'-DGIT_VERSION="$(GIT_VERSION)"' \
+	'-DGIT_USER_AGENT=$(GIT_USER_AGENT_CQ_SQ)'
 
 $(BUILT_INS): git$X
 	$(QUIET_BUILT_IN)$(RM) $@ && \
@@ -2710,6 +2716,7 @@ 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
 
 .PHONY: all install profile-clean clean strip
 .PHONY: shell_compatibility_test please_set_SHELL_PATH_to_a_more_modern_shell