Respect CFLAGS.

Index: Makefile.in
--- Makefile.in.orig
+++ Makefile.in
@@ -15,7 +15,7 @@ inst_dir := install -m 755 -d
 inst_file := install -m 644
 
 CC := @CC@
-CFLAGS := @CFLAGS@ -pthread -Isrc/include -Imlk/include -I. -DHAVE_CONFIG_H
+CFLAGS += @CFLAGS@ -pthread -Isrc/include -Imlk/include -I. -DHAVE_CONFIG_H
 LDFLAGS := @LDFLAGS@ -pthread
 LIBS := @LIBS@ -lm
 
