scons: Support MinGW32 cross compiler.

To build an alternative opengl32.dll with Gallium's software-rasterizer from a
debian-based distribution run:

  sudo apt-get install mingw32
  scons platform=windows toolchain=crossmingw machine=x86 winsys=gdi dri=no

common.py

@@ -60,6 +60,8 @@ def AddOptions(opts):
 											 allowed_values=('generic', 'ppc', 'x86', 'x86_64')))
 	opts.Add(EnumOption('platform', 'target platform', default_platform,
 											 allowed_values=('linux', 'cell', 'windows', 'winddk', 'wince')))
+	opts.Add(EnumOption('toolchain', 'compiler toolchain', 'default',
+											 allowed_values=('default', 'crossmingw')))
 	opts.Add(BoolOption('llvm', 'use LLVM', 'no'))
 	opts.Add(BoolOption('dri', 'build DRI drivers', default_dri))