makefile: allow compiler to be set and use gcc only as default

makefile

@@ -38,7 +38,9 @@ ifeq ($(filter $(MAKECMDGOALS),clean help),)
     endif
 endif
 
-### Recommended gcc flags
+CC ?= gcc
+
+### Recommended compiler flags
 CFLAGS         = -std=gnu99
 CFLAGS        += -Os
 CFLAGS        += -pedantic
@@ -60,10 +62,10 @@ CFLAGS        += -Wunreachable-code
 ### all ###
 all: $(OBJS) $(CURDIR)/$(TARGET).elf
 $(CURDIR)/$(TARGET).elf: $(OBJS) | $(BUILDDIR)
-	gcc $(CFLAGS) $(INCS) -o $(CURDIR)/$(TARGET).elf $(OBJS)
+	$(CC) $(CFLAGS) $(INCS) -o $(CURDIR)/$(TARGET).elf $(OBJS)
 
 $(BUILDDIR)/%.o: $(SRCDIR)/%.c | $(BUILDDIR)
-	gcc -c $(CFLAGS) $(INCS) $< -o $@
+	$(CC) -c $(CFLAGS) $(INCS) $< -o $@
 
 $(BUILDDIR):
 	mkdir -p $(BUILDDIR)