Changing the devkitPro path from absolute to environment defined

### Problem

The user mgiht want to place devkitPro in f.e. "/c/files/source/lib/devkitpro" instead of "/c/devkitpro"

### Solution

Change "C:\devkitPro" to "$(DEVKITPRO_)" because

* that allow VS to locate function declarations

The environment variable is "DEVKITPRO_" instead of "DEVKITPRO" because devkitPro is designed to build from MSYS where a Windows style path is unhandled when used like this

	# start devkitPro MSYS environment
	call c:\files\source\lib\devkitpro\msys\msys.bat -mintty MSYS

	# environment information
	uname -a; echo $MSYSTEM
	MSYS_NT-6.1 PC 1.0.17(0.48/3/2) 2011-04-24 23:39 i686 Msys
	MSYS

	# set Windows style devkitPro path
	export DEVKITPRO=c:\\files\\source\\lib\\devkitpro
	export DEVKITPPC=$DEVKITPRO\\devkitppc

	# build WPAD
	cd /c/file/source/dolphin/repo/Source/TestSuite/WPAD
	make -r
		  0 [main] make 22152 open_stackdumpfile: Dumping stack trace to make.exe.stackdump

	# set unix style devkitPro path
	export DEVKITPRO=/c/files/source/lib/devkitpro
	export DEVKITPPC=$DEVKITPRO/devkitppc

	# build WPAD
	make -r
	dolphintest_wpad.cpp
	linking ... WPAD.elf
	output ... WPAD.dol