This should allow users to pick which llvmPackages they would like to
use, since Klee supports llvmPackages_11 through llvmPackages_16 as of
Klee 3.1 (defaulting to 13 for stability).
Use parallel building for both Klee and klee-uclibc to make things faster.
with structuredAttrs lists will be bash arrays which cannot be exported
which will be a issue with some patches and some wrappers like cc-wrapper
this makes it clearer that NIX_CFLAGS_COMPILE must be a string as lists
in env cause a eval failure
checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
