Tweak Makefile

* Use well-known variable names
* Respect values set in environment

Obtained from FreeBSD ports collection

Makefile

@@ -1,22 +1,22 @@
-GCC=gcc
-CCFLAGS=-Wall -pedantic -std=c99 -D_BSD_SOURCE
-LDFLAGS=-lImlib2
+CC?=gcc
+CFLAGS+=-Wall -pedantic -std=c99 -D_BSD_SOURCE
+LDFLAGS+=-lImlib2
 PROG=icat
 MODS=icat.o
 
 uname_S := $(shell sh -c 'uname -s 2>/dev/null || echo unknown')
 
 ifeq ($(uname_S),Darwin)
-	CCFLAGS:=$(CCFLAGS) -I/opt/X11/include
+	CFLAGS:=$(CFLAGS) -I/opt/X11/include
 endif
 
 all: $(PROG)
 
 %.o: %.c
-	$(GCC) -c $(CCFLAGS) -o $@ $<
+	$(CC) -c $(CFLAGS) -o $@ $<
 
 $(PROG): $(MODS)
-	$(GCC) -o $@ $< $(LDFLAGS)
+	$(CC) -o $@ $< $(LDFLAGS)
 
 clean:
 	-rm -f $(PROG) $(MODS)