Revert "Move the glxfbconfig configtest to qpa and rename it to glx"

This reverts commit a2d3b7c99165cb5c4be6f0dd83a967e1119cf732. That
commit broke -opengl es2 builds on Linux desktops (when GLX is
available).

/usr/include/GL/gl.h:162:17: error: conflicting declaration ‘typedef double GLdouble’
qopengl.h:97:17: error: ‘GLdouble’ has a previous declaration as ‘typedef GLfloat GLdouble’

Change-Id: Id5f48c05803be1a8c03eeffca139b80e9a85a0eb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Don't check the Qt version across modules in -developer-build

Instead of enforcing the check, we'll simply trust developers to do the
right thing. For one, it's useful to mix libraries during testing
(regression testing, git bisects, etc.). For another, Qt developers are
faced day-to-day with binary incompatibility issues anyway, so this
check is mostly superfluous.

If there's one commit we could be sure that isn't breaking binary
compatibility, that's the "Bump Qt version" commit. And yet that's the
one that would cause the fatal to trip...

Change-Id: I8965f764a6ca1b2d125b42bce7ac6b27e3afc8ac
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>