Honor CPPFLAGS and LDFLAGS.

Makefile

@@ -1,6 +1,6 @@
 CFLAGS = -ggdb -Wall -Wextra -Wshadow -Wformat-security -O2
 all: filejoinfs
 filejoinfs: filejoinfs.c
-	${CC} ${CFLAGS} `pkg-config fuse --cflags --libs` filejoinfs.c -lfuse -o $@
+	${CC} ${CPPFLAGS} ${CFLAGS} ${LDFLAGS} `pkg-config fuse --cflags --libs` filejoinfs.c -lfuse -o $@
 clean:
 	rm filejoinfs