configure.ac: explicitly require libdrm for dri classic drivers.
configure.ac: explicitly require libdrm for dri classic drivers.

Although this might come from somewhere else require it explicitly.

Reviewed-by: Chad Versace <4b94d436ff8c93527a96d251ac64b06311506efe@chromium.org>
Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler