They are not use by default since 2011, and non of the distro configs
uses --without-system-xextensions-headers.

Change-Id: I51e88796c22b1b3d0854c3ec1db15fcab720a079
Reviewed-on: https://gerrit.libreoffice.org/5175Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>