VS install found through --with-tools-dir value is discarded

[Jorn Vernee]

Hi,

I was testing with different versions of Visual Studio to try and nail 
down the source of a deprecation warning. I was using the 
--with-tools-dir config option to quickly switch between installations 
but noticed the VS install that was being found through that method was 
being discarded, leading to a failed configuration.

The fix is pretty simple: 
http://cr.openjdk.java.net/~jvernee/vs_tools_dir/webrev.00/

The TOOLCHAIN_CHECK_POSSIBLE_VISUAL_STUDIO_ROOT macro sets the 
VS_ENV_CMD  var, and skips doing anything when it is already set. So, 
previously, by setting it to an empty string _after_ the check for 
--with-tools-dir any VS install that's found with that method is always 
discarded, or otherwise overwritten when another method for finding a VS 
installation also works.

With that fix selecting the VS installation works as expected.

On a side note; I noticed that the hostx86 toolchain is preferred over 
the hostx64 version in VS 2017+, due to the order of values in 
VCVARSFILES in TOOLCHAIN_CHECK_POSSIBLE_VISUAL_STUDIO_ROOT. Is there any 
particular reason for this? I'd expect the x64 toolchain would be 
preferred on x64 platforms.

Jorn