GCC: Set LANG=C

Set LANG=C when building with GCC. This can be overridden in the
environment section of the project build settings.

This change should make sure that Qt Creator is able to properly
parse build issues.

Task-number: QTCREATORBUG-4011
Change-Id: Ibff57feff7945fc7e03acca3a86323b63c9d66ae
Reviewed-on: http://codereview.qt.nokia.com/3193


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>