Eliminate spurious compiler warnings from system headers on *BSD platforms.

Ensure that system headers located in /usr/local/include are actually treated
as system headers by the compiler, and not as local headers which are subject
to warnings from the -Wundef compiler option and others.

(closes issue #15606)
Reported by: mvanbaak



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@210237 65c4cc65-6c06-0410-ace0-fbb531ad65f3

Makefile

@@ -221,7 +221,7 @@ ifeq ($(AST_DEVMODE),yes)
 endif
 
 ifneq ($(findstring BSD,$(OSARCH)),)
-  _ASTCFLAGS+=-I/usr/local/include
+  _ASTCFLAGS+=-isystem /usr/local/include
   _ASTLDFLAGS+=-L/usr/local/lib
 endif