--- Makefile.orig
+++ Makefile
@@ -62,7 +62,7 @@
 
 #### Installation options.
 DESTDIR =
-PREFIX = /usr/local
+PREFIX? = /usr/local
 SBINDIR = $(PREFIX)/sbin
 MANDIR = $(PREFIX)/share/man
 DOCDIR = $(PREFIX)/doc/haproxy
@@ -89,7 +89,7 @@
 
 #### Toolchain options.
 # GCC is normally used both for compiling and linking.
-CC = gcc
+CC? = gcc
 LD = $(CC)
 
 #### Debug flags (typically "-g").
@@ -158,7 +158,7 @@
 # These CFLAGS contain general optimization options, CPU-specific optimizations
 # and debug flags. They may be overridden by some distributions which prefer to
 # set all of them at once instead of playing with the CPU and DEBUG variables.
-CFLAGS = $(ARCH_FLAGS) $(CPU_CFLAGS) $(DEBUG_CFLAGS) $(SPEC_CFLAGS)
+CFLAGS? = $(ARCH_FLAGS) $(CPU_CFLAGS) $(DEBUG_CFLAGS) $(SPEC_CFLAGS)
 
 #### Common LDFLAGS
 # These LDFLAGS are used as the first "ld" options, regardless of any library