respect env CPPFLAGS

--- a/src/Makefile
+++ b/src/Makefile
@@ -2,8 +2,9 @@ prefix=/usr
 includedir=$(prefix)/include
 libdir=$(prefix)/lib
 
-CFLAGS ?= -g -fomit-frame-pointer -O2
-CFLAGS += -Wall -I. -fPIC
+CFLAGS ?= -fomit-frame-pointer -O2
+CFLAGS += -I. -fPIC
+CFLAGS += $(CPPFLAGS)
 SO_CFLAGS=-shared $(CFLAGS)
 L_CFLAGS=$(CFLAGS)
 LINK_FLAGS=

