crystal: fix build with newer versions of clang

Crystal requires linking libc++abi when building with newer versions of
clang. See NixOS#166205.

pkgs/development/compilers/crystal/default.nix

@@ -148,6 +148,11 @@ let
         # See https://github.com/NixOS/nixpkgs/pull/195606#issuecomment-1356491277
         substituteInPlace spec/compiler/loader/unix_spec.cr \
           --replace 'it "parses file paths"' 'pending "parses file paths"'
+      '' + lib.optionalString (stdenv.cc.isClang && (stdenv.cc.libcxx != null)) ''
+        # Darwin links against libc++ not libstdc++. Newer versions of clang (12+) require
+        # libc++abi to be linked explicitly (see https://github.com/NixOS/nixpkgs/issues/166205).
+        substituteInPlace src/llvm/lib_llvm.cr \
+          --replace '@[Link("stdc++")]' '@[Link("c++", "-l${stdenv.cc.libcxx.cxxabi.libName}")]'
       '';
 
       # Defaults are 4