clover: Assorted build fixes.

clover: Assorted build fixes.

Contains the following patches squashed in:

commit 9fff1dc0875f7c9591550fa3ebbe1ba7a18483fa
Author: Tom Stellard <thomas.stellard@amd.com>
Date:   Tue Mar 20 23:20:03 2012 +0100

    configure.ac: Build gallium loader when OpenCL is enabled

commit 542111cb02957418c6a285cb6ef2924e49adc66e
Author: Tom Stellard <thomas.stellard@amd.com>
Date:   Tue Mar 20 23:30:29 2012 +0100

    configure.ac: Add sw/null to GALLIUM_WINSYS_DIRS for gallium loader

commit 876f8de46062dde76b6075be3b6628f969b16648
Author: Tom Stellard <thomas.stellard@amd.com>
Date:   Thu Feb 9 11:26:05 2012 -0500

    configure.ac: Require gcc > 4.6.0 for clover

commit 99049d50fa3d9a23297ae658189c19c89dca1766
Author: Tom Stellard <thomas.stellard@amd.com>
Date:   Tue Mar 20 23:32:06 2012 +0100

    configure.ac: Require Gallium drm loader when gallium loader is enabled

    No longer silently exclude this when building OpenCL drivers
    for nouveau and r600.