Bug#877359: clang: precompiled headers are not reproducible

[Rebecca N. Palmer]

Possible workarounds for beignet:

- Remove the .pch files entirely.  This slows down compiling (which is a 
run-time operation in OpenCL): 263sec instead of 90sec for the 
540-kernel test suite = ~0.3sec extra per kernel compile.  At a guess, 
that's unlikely but not impossible to matter in practice.

- Generate the .pch files at install time instead of build time.  That 
means adding a dependency on clang (~60MB download size), and if done 
the obvious way, running it as root (which might be worse for security 
than staying unreproducible).

Worth it or not?  (beignet will soon need an upload anyway, for #873403.)