Add build support for iwyu

include-what-you-use is a tool to help removing unneeded includes.
Homepage:
https://code.google.com/p/include-what-you-use/

So to use it pass its path to configure like:
./configure --with-iwyu=/usr/bin/include-what-you-use

May be helpful for fdo#42949

Change-Id: Idc185c5181d754b9dfd82fcf6a5ad05953b3cd03
Reviewed-on: https://gerrit.libreoffice.org/12255
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>