Is your feature request related to a problem? Please describe.
When using llvm as a library, an application using llvm fails with a linker error, specifically:
CMakeFiles/SPIRVToLLVM.dir/SpirvJit.cpp.o:(.data.rel.ro._ZTIN4llvm9ErrorInfoINS_9ErrorListENS_13ErrorInfoBaseEEE[_ZTIN4llvm9ErrorInfoINS_9ErrorListENS_13ErrorInfoBaseEEE]+0x10): undefined reference to typeinfo for llvm::ErrorInfoBase

Proposed solution
Add -DLLVM_ENABLE_RTTI=1 to the portfile.