LLVM's build system creates NATIVE/bin/llvm-config by reexecuting cmake
with entirely new flags. Problematically, the `CMAKE_INSTALL_*` flags
are not inherited, causing llvm-config-native to return wrong
installation paths, e. g. CMAKE_INSTALL_LIBDIR would default to `lib64`
on x86_64-linux. Previously this was masked by outputs.patch which
replaced ActiveLibDir with a string passed in from Nix, however
`--cmakedir` for example would turn out to be wrong always, breaking
cross-compilation of e. g. lld.
Additionally LLVM_ENABLE_RTTI needs to be repassed, as it is used to
determine if RTTI is available. Passing LLVM_LINK_LLVM_DYLIB is crucial
if we are building LLVM non-statically: It influences the --shared-mode
flag (which should indicate that -lLLVM is enough to link all
components) and makes --link-shared work in the first place,
i. e. llvm-config-native believes the built shared libs don't exist
unless we repass this flag.
Passing LLVM_LINK_LLVM_DYLIB=ON, however, makes the native build produce
a full libLLVM.so which is something we don't want, so we introduce a
patch which forces llvm-config to link statically against the LLVM
components it needs.
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