Fixes and improvements to support Precompiled Headers
on Linux.

Change-Id: I8145c146b0dba56c7a4d0fdf9c330164b67ada53
Reviewed-on: https://gerrit.libreoffice.org/21307Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>