Fix some minor build system annoyances

[rzumer]

I found it mildly annoying to have to run some steps manually before and after the main build, so this PR automates these steps for non-ARM builds (on ARM, MSBuild pre-/post-build steps don't seem to be supported).

It also fixes an issue in the dependency build script where the environment is modified for the whole session, which can cause it to fail due to vcvarsall trying to make (I assume) PATH too long when called multiple times.