Delete .cmd scripts created by Boost

[zackgalbreath]

Boost creates .cmd scripts to set up the PATH to point to cl.exe. These are named something like
C:\Users\<myusername>\AppData\Local\Temp\b2_msvc_14.1_vcvarsx86_amd64_.cmd

A Visual Studio upgrade moves cl.exe to a new location, causing this .cmd script to become stale & incorrectly set the path. This causes b2 to fail because it can't find cl.exe.

To solve this problem, we should teach Fletch to delete these files early in the configure step of Boost. We can use the TEMP environment variable to get the location of the directory containing these scripts.

[dstoup]

This is a better solution for sure. I have bumped into this issue on upgrade before as have others. Fixing this issue will save people a lot of time.

A note to anyone who looks at this issue before I can, we should probably target the release branch.