Add quotes to INCLUDE/LIB paths as in Phobos win64.mak

This not only brings the 2 in line w.r.t to quotes, it also properly allows overriding of environment.

As per Walter's hint I use the following makefile side by side to set my env.
```
VCDIR=\Program Files (x86)\Microsoft Visual Studio 11.0\VC
SDKDIR=\Program Files (x86)\Microsoft SDKs\Windows\v8.0A

all:	
	make -f win64.mak VCDIR="$(VCDIR)" SDKDIR="$(SDKDIR)"

clean:
	make -f win64.mak clean

```
This fails with druntime's makefile but works just fine with Phobos one.