loader: introduce the loader util lib

All the various window system integration layers duplicate roughly the
same code for figuring out device and driver name, pci-id's, etc.  Which
is sad.  So extract it out into a loader util lib.

v2 (Emil)
* Separate the introduction of libloader from the code de-duplication.
* Strip out non-pci devices support.
* Add scons + Android build system support.
* Add VISIBILITY_CFLAGS to avoid exporting the loader funcs.

v3 (Emil)
* PIPE_OS_ANDROID is undefined at this scope, use ANDROID
* Make sure we define _EGL_NO_DRM when building only swrast

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/SConscript

@@ -11,6 +11,7 @@ if env['hostonly']:
     # compilation
     Return()
 
+SConscript('loader/SConscript')
 
 # When env['gles'] is set, the targets defined in mapi/glapi/SConscript are not
 # used.  libgl-xlib and libgl-gdi adapt themselves to use the targets defined