swr: [scons] Fix windows build
swr: [scons] Fix windows build

Fix codegen build break that was introduced earlier

v2: update rules for gen_knobs.cpp and gen_knobs.h

v3: Introduce bldroot and revert generator file changes, making patch simpler.

Reviewed-by: Tim Rowley <54947742fe7299a578dbb84f6c782c6c4c668d8f@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler