Use CC variable from environment, don't hardcode "cc".

--- glew/config/Makefile.linux.orig	2014-01-29 15:39:43.364972925 +0100
+++ glew/config/Makefile.linux	2014-01-29 15:44:45.621000941 +0100
@@ -1,6 +1,6 @@
 NAME = $(GLEW_NAME)
-CC = cc
-LD = cc
+CC ?= cc
+LD ?= $(CC)
 M_ARCH ?= $(shell uname -m)
 ARCH64 = false
 ifeq (x86_64,${M_ARCH})
--- glew/config/Makefile.darwin.orig	2014-01-29 15:44:39.541000377 +0100
+++ glew/config/Makefile.darwin	2014-01-29 15:44:47.548001120 +0100
@@ -1,6 +1,6 @@
 NAME = $(GLEW_NAME)
-CC = cc
-LD = cc
+CC ?= cc
+LD ?= $(CC)
 CFLAGS.EXTRA = -dynamic -fno-common
 #CFLAGS.EXTRA += -no-cpp-precomp
 LDFLAGS.EXTRA =