Support mxe build environment for windows cross builds

Override all tools with prefixed versions if the CROSS variable has
been set. This is the recommended way to enable cross compilation using
mxe without touching lots of environment variables first.

common.mk

@@ -1,9 +1,17 @@
 # makefile variables and rules shared by both gkrellm and gkrellmd
 
-CC ?= gcc
-AR ?= ar
-PKG_CONFIG ?= pkg-config
-WINDRES ?= windres
+ifdef CROSS
+	# enforce overriding tools for mxe cross builds (http://mxe.cc)
+	CC = $(CROSS)gcc
+	AR = $(CROSS)ar
+	PKG_CONFIG = $(CROSS)pkg-config
+	WINDRES = $(CROSS)windres
+else
+	CC ?= gcc
+	AR ?= ar
+	PKG_CONFIG ?= pkg-config
+	WINDRES ?= windres
+endif
 
 PREFIX ?= /usr/local
 INSTALLROOT ?= $(DESTDIR)$(PREFIX)