scons: Add swr compile option

To buils The SWR driver (currently optional, not compiled by default)

v3: add option as opposed to target

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

common.py

@@ -110,5 +110,6 @@ def AddOptions(opts):
     opts.Add(BoolOption('texture_float',
                         'enable floating-point textures and renderbuffers',
                         'no'))
+    opts.Add(BoolOption('swr', 'Build OpenSWR', 'no'))
     if host_platform == 'windows':
         opts.Add('MSVC_VERSION', 'Microsoft Visual C/C++ version')