Prepare build on macOS

[ttuegel]

This pull request clears up the remaining issues with building the backend on macOS except that we still need to update macOS SDK version upstream in Nixpkgs (NixOS/nixpkgs#101229). Still, I don't want to lose the progress we have made so far.

[ttuegel]

The build is failing with this error:

cd /tmp/nix-build-llvm-backend-0.drv-0/llvm-backend/build/runtime/configurationparser && /nix/store/ff3sk8lzq1xga26zyd5ghwz5qra3ian1-clang-wrapper-10.0.0/bin/c++   -I/tmp/nix-build-llvm-backend-0.drv-0/llvm-backend/include -I/tmp/nix-build-llvm-backend-0.drv-0/llvm-backend/build/include  -fno-stack-protector -g -O3 -DNDEBUG -arch x86_64   -Wall -Werror -Wno-return-type-c-linkage -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -std=c++17 -o CMakeFiles/configurationparser.dir/ConfigurationParser.cpp.o -c /tmp/nix-build-llvm-backend-0.drv-0/llvm-backend/runtime/configurationparser/ConfigurationParser.cpp
/tmp/nix-build-llvm-backend-0.drv-0/llvm-backend/runtime/configurationparser/ConfigurationParser.cpp:32:80: error: 'get<const kllvm::KOREPattern *, const kllvm::KOREPattern *, construction>' is unavailable: introduced in macOS 10.14
      const auto constructor = dynamic_cast<const KORECompositePattern *>(std::get<const KOREPattern *>(current));
                                                                               ^
/nix/store/w7x7s9g7lrzzds5b7i0q5sjbskvqpimy-libc++-10.0.0/include/c++/v1/variant:1429:16: note: 'get<const kllvm::KOREPattern *, const kllvm::KOREPattern *, construction>' has been explicitly marked unavailable here
constexpr _Tp& get(variant<_Types...>& __v) {
               ^
/tmp/nix-build-llvm-backend-0.drv-0/llvm-backend/runtime/configurationparser/ConfigurationParser.cpp:62:27: error: 'get<construction, const kllvm::KOREPattern *, construction>' is unavailable: introduced in macOS 10.14
      uint32_t tag = std::get<construction>(current).tag;
                          ^
/nix/store/w7x7s9g7lrzzds5b7i0q5sjbskvqpimy-libc++-10.0.0/include/c++/v1/variant:1429:16: note: 'get<construction, const kllvm::KOREPattern *, construction>' has been explicitly marked unavailable here
constexpr _Tp& get(variant<_Types...>& __v) {
               ^
/tmp/nix-build-llvm-backend-0.drv-0/llvm-backend/runtime/configurationparser/ConfigurationParser.cpp:63:31: error: 'get<construction, const kllvm::KOREPattern *, construction>' is unavailable: introduced in macOS 10.14
      size_t nchildren = std::get<construction>(current).nchildren;
                              ^
/nix/store/w7x7s9g7lrzzds5b7i0q5sjbskvqpimy-libc++-10.0.0/include/c++/v1/variant:1429:16: note: 'get<construction, const kllvm::KOREPattern *, construction>' has been explicitly marked unavailable here
constexpr _Tp& get(variant<_Types...>& __v) {
               ^
3 errors generated.

According to NixOS/nixpkgs#101229, we are using the macOS 10.12 SDK, which probably explains this error.