macro_fallback with many headers fails because pch file name is too long

[corecode]

When generating bindings for many headers, try_ensure_fallback_translation_unit fails because pch is constructed with a excessively long filename. Replacing it just by "precompile.h.pch" fixes this.

[corecode]

A workaround is to create a header full of #includes and then only using .header() for that one file.