Link libunwind properly on macos (#1093)

On macOS, we need to look for `libunwind` as an LLVM library, rather
than a system-installed dependency. This means adding some additional
logic to the logic that assembles compiler flags when invoking `clang`;
we can't directly look for `@BREW_PREFIX@/opt/llvm/...` because we don't
necessarily know what formula version the user has installed.

Author: Baltoli

bin/llvm-kompile-clang

@@ -136,6 +136,7 @@ if [[ "$OSTYPE" == "darwin"* ]]; then
   flags=(
     "-L@BREW_PREFIX@/opt/libffi/lib"
     "-L@BREW_PREFIX@/lib"
+    "-L@LLVM_LIBRARY_DIR@"
     "-Wl,-u,_table_getArgumentSortsForTag"
     "-I" "@BREW_PREFIX@/include"
   )

cmake/FindLLVM.cmake

@@ -29,6 +29,11 @@ find_program(LLDB lldb
   PATHS ${LLVM_TOOLS_BINARY_DIR}
   NO_DEFAULT_PATH)
 
+execute_process(
+  COMMAND "${LLVM_TOOLS_BINARY_DIR}/llvm-config" "--libdir"
+  OUTPUT_VARIABLE LLVM_LIBRARY_DIR
+  OUTPUT_STRIP_TRAILING_WHITESPACE)
+
 if(NOT OPT)
   message(FATAL_ERROR "Could not find an opt binary. Is llvm installed on your PATH?")
 endif()

nix/llvm-backend.nix

@@ -42,6 +42,7 @@ stdenv.mkDerivation {
       --replace '"-ltinfo"' '"-L${ncurses}/lib" "-ltinfo"' \
       --replace '"-lunwind"' '"-L${libunwind}/lib" "-lunwind"' \
       --replace '"-L@BREW_PREFIX@/opt/libffi/lib"' ' ' \
+      --replace '"-L@LLVM_LIBRARY_DIR@"' ' ' \
       --replace '-L@BREW_PREFIX@/lib' '-L${libcxx}/lib' \
       --replace '-I "$(dirname "$0")"/../include/kllvm' \
                 '-I "$(dirname "$0")"/../include/kllvm -I ${boost.dev}/include -I ${fmt.dev}/include'