build fix: external CFLAGS are ignored

* When passing CFLAGS to ./configure, they are ignored.
* The reason is that in Makefile.am CFLAGS are clobbered.

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>

Makefile.am

@@ -5,7 +5,7 @@ LEGACY_MAKE	= \
 		top_srcdir=$(top_srcdir) \
 		srcdir=$(srcdir)
 
-CFLAGS	= -g -Wall -O2 $(DAHDI_INCLUDE)
+CFLAGS	+= -g -Wall -O2 $(DAHDI_INCLUDE)
 if DAHDI_DEVMODE
 CFLAGS	+= \
 	-Werror \