Allow a pre-computed checksum file for precompiled headers

Allow a pre-computed checksum file for precompiled headers

Having large precompiled header files slows down direct mode off
CCache considerably. Allow CCache to check against a (much smaller)
pre-computed checksum file instead of the precompiled header file
itself. This checksum file will be used in the manifest instead of
the actual precompiled header file.

Note that to make this to work, the build system needs to keep a
checksum file in sync with the precompiled header.

The behavior can be activated by the `pch_external_checksum` option.