Every time ninja invoked CMake, we were unconditionally regenerating
all of our "licenses.inc" files which caused a ton of re-compiles.
Before the change, building after touching `ubershader.mat.in` took over
a minute, now it takes 20 seconds.
The CMake "file" commands are bad. The CMake documentation says:
If the file is a build input, use the configure_file() command
to update the file only when its content changes.
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