scons: add 'targets' variable, for specifying ... targets

Ideally scons should be able to work backwards from the list of
targets to figure out which drivers, state trackers and other
convenience libraries need to be built.

SConstruct

@@ -31,6 +31,7 @@ import common
 # Configuration options
 
 default_statetrackers = 'mesa'
+default_targets = 'none'
 
 if common.default_platform in ('linux', 'freebsd', 'darwin'):
 	default_drivers = 'softpipe,failover,svga,i915,i965,trace,identity,llvmpipe'
@@ -54,6 +55,28 @@ opts.Add(ListVariable('drivers', 'pipe drivers to build', default_drivers,
 opts.Add(ListVariable('winsys', 'winsys drivers to build', default_winsys,
                      ['xlib', 'vmware', 'i915', 'i965', 'gdi', 'radeon', 'graw-xlib']))
 
+opts.Add(ListVariable('targets', 'driver targets to build', default_targets,
+		      ['dri-i915',
+		       'dri-i965',
+		       'dri-nouveau',
+		       'dri-radeong',
+		       'dri-swrast',
+		       'dri-vmwgfx',
+		       'egl-i915',
+		       'egl-i965',
+		       'egl-nouveau',
+		       'egl-radeon',
+		       'egl-swrast',
+		       'egl-vmwgfx',
+		       'graw-xlib',
+		       'libgl-gdi',
+		       'libgl-xlib',
+		       'xorg-i915',
+		       'xorg-i965',
+		       'xorg-nouveau',
+		       'xorg-radeon',
+		       'xorg-vmwgfx']))
+
 opts.Add(EnumVariable('MSVS_VERSION', 'MS Visual C++ version', None, allowed_values=('7.1', '8.0', '9.0')))
 
 env = Environment(