Honor AR, CC and CXX from the user enviroment instead of relying on
${CHOST}-driver though it may be correct most of the time.
Author: Lucio Sauer <watermanpaint@posteo.net>
--- a/Makefile
+++ b/Makefile
@@ -27,13 +27,13 @@ CROSS_PREFIX:=x86_64-w64-mingw32-
 #CROSS_PREFIX=i686-w64-mingw32-
 EXE:=.exe
 else
-CROSS_PREFIX:=
+CROSS_PREFIX:=${CHOST}-
 EXE:=
 endif
 
-CC=$(CROSS_PREFIX)gcc
-CXX=$(CROSS_PREFIX)g++
-AR=$(CROSS_PREFIX)ar
+CC ?= $(CROSS_PREFIX)gcc
+CXX ?= $(CROSS_PREFIX)g++
+AR ?= $(CROSS_PREFIX)ar
 EMCC=emcc
 
 PWD:=$(shell pwd)