--- silo-git.orig/Rules.make	2017-10-14 10:02:18.000000000 +0200
+++ silo-git/Rules.make	2017-10-15 11:42:54.792574664 +0200
@@ -3,12 +3,12 @@ IMGVERSION=0.99
 SHELL=/bin/bash
 RM=rm -f
 # We want to force 32-bit builds
-CC=gcc -m32
-HOSTCC=gcc
-LD=ld -m elf32_sparc
-AS=as
-STRIP=strip
-NM=nm
+CC?=gcc -m32
+HOSTCC?=gcc
+LD?=ld -m elf32_sparc
+AS?=as
+STRIP?=strip
+NM?=nm
 ELFTOAOUT=elftoaout
 BIN2H=../common/bin2h
 
@@ -18,7 +18,8 @@ LARGE_RELOC=0x380000
 cc-option-yn = $(shell if $(CC) $(CFLAGS) $(1) -S -o /dev/null -xc /dev/null \
 		> /dev/null 2>&1; then echo "y"; else echo "n"; fi;)
 
-CFLAGS = -Os -Wall -I. -I../include -fomit-frame-pointer \
+CFLAGS ?=
+CFLAGS += -Wall -I. -I../include -fomit-frame-pointer \
 	-fno-strict-aliasing -DSMALL_RELOC=$(SMALL_RELOC) \
 	-DLARGE_RELOC=$(LARGE_RELOC)
 HOSTCFLAGS = -O2 -Wall