Merge branch 'ps/make-append-to-cflags'

To help developers, the build procedure now allows builders to use
CFLAGS_APPEND to specify additional CFLAGS.

* ps/make-append-to-cflags:
  Makefile: add ability to append to CFLAGS and LDFLAGS

Author: gitster

Makefile

@@ -1449,8 +1449,8 @@ ALL_COMMANDS_TO_INSTALL += git-upload-archive$(X)
 ALL_COMMANDS_TO_INSTALL += git-upload-pack$(X)
 endif
 
-ALL_CFLAGS = $(DEVELOPER_CFLAGS) $(CPPFLAGS) $(CFLAGS)
-ALL_LDFLAGS = $(LDFLAGS)
+ALL_CFLAGS = $(DEVELOPER_CFLAGS) $(CPPFLAGS) $(CFLAGS) $(CFLAGS_APPEND)
+ALL_LDFLAGS = $(LDFLAGS) $(LDFLAGS_APPEND)
 
 ifdef SANITIZE
 SANITIZERS := $(foreach flag,$(subst $(comma),$(space),$(SANITIZE)),$(flag))