--- simh-3.9.0/makefile
+++ simh-3.9.0/makefile
@@ -300,8 +300,8 @@
   CFLAGS_O = -O0
   BUILD_FEATURES = - debugging support
 else
-  CFLAGS_O = -O2
-  LDFLAGS_O = 
+  CFLAGS_O ?= -O2
+  LDFLAGS_O ?= 
   GCC_MAJOR_VERSION = $(firstword $(subst  ., ,$(GCC_VERSION)))
   ifneq (3,$(GCC_MAJOR_VERSION))
     ifeq (,$(GCC_OPTIMIZERS_CMD))