use CPPFLAGS if set

CPPFLAGS is a traditional build environment variable used to set C
pre-processor flags.  Accept these flags during the build if they've
been set.

config.mk

@@ -34,7 +34,7 @@ INSTALL := install
 
 # Flags
 BUILD_LDFLAGS += $(LDFLAGS)
-BUILD_CFLAGS := $(CFLAGS) -std=c99 -D_GNU_SOURCE
+BUILD_CFLAGS := $(CFLAGS) $(CPPFLAGS) -std=c99 -D_GNU_SOURCE
 BUILD_CFLAGS += -Wno-unused -Wtype-limits -Wformat -Wformat-security -Wall
 BUILD_CFLAGS += -I$(abspath .) -I$(abspath lib/generic) -I$(abspath contrib)
 BUILD_CFLAGS += -DPACKAGE_VERSION="\"$(VERSION)\"" -DPREFIX="\"$(PREFIX)\"" -DMODULEDIR="\"$(MODULEDIR)\""