Based on the LLVM documentation [1], Python is used to run the automated
test suite. Therefore is it always a dependency for LLVM. However, if
build without Python (~python), we limit it to a build time dependency.
Note that py-lit is not added as a spack dependency even though it is
available as a spack package. This is because it is already included
in llvm and llvm is difficult to configure using an external py-lit
(several CMake variables to set correctly). Additionally, having
py-lit as a spack dependency adds Python as a runtime dependency
for llvm even though it is not required at runtime.
[1] https://llvm.org/docs/GettingStarted.html#requirements
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