[v3] tweaks, remove pch install rules

[Benjamin Kosnik]

This removes install rules for PCH files, and instead installs the
files that are used when creating the binary PCH files, ie *.gch. The
thought is that these files are actually more useful for people than a
binary blob that may be generated with the wrong flags/include/etc, in
that at least re-generating the PCH files with the locally blessed
flags is then possible.

Also, a collection of unrelated, random tweaks from various side branches.

tested x86/linux

-benjamin
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