meson: Fix linking ObjC objects on Darwin

https://github.com/NixOS/nixpkgs/pull/54007#issuecomment-454693658

pkgs/development/tools/build-managers/meson/default.nix

@@ -44,6 +44,13 @@ python3Packages.buildPythonApplication rec {
       src = ./fix-rpath.patch;
       inherit (builtins) storeDir;
     })
+  ] ++ lib.optionals stdenv.isDarwin [
+    # We use custom Clang, which makes Meson think *not Apple*, while still
+    # relying on system linker. When it detects standard Clang, Meson will
+    # pass it `-Wl,-O1` flag but optimizations are not recognized by
+    # Mac linker.
+    # https://github.com/mesonbuild/meson/issues/4784
+    ./fix-objc-linking.patch
   ];
 
   setupHook = ./setup-hook.sh;

pkgs/development/tools/build-managers/meson/fix-objc-linking.patch

@@ -0,0 +1,20 @@
+--- a/mesonbuild/environment.py
++++ b/mesonbuild/environment.py
+@@ -795,7 +795,7 @@
+                 compiler_type = self.get_gnu_compiler_type(defines)
+                 version = self.get_gnu_version_from_defines(defines)
+                 return GnuObjCCompiler(ccache + compiler, version, compiler_type, is_cross, exe_wrap, defines)
+-            if out.startswith('Apple LLVM'):
++            if out.startswith('Apple LLVM') or mesonlib.for_darwin(want_cross, self):
+                 return ClangObjCCompiler(ccache + compiler, version, CompilerType.CLANG_OSX, is_cross, exe_wrap)
+             if out.startswith('clang'):
+                 return ClangObjCCompiler(ccache + compiler, version, CompilerType.CLANG_STANDARD, is_cross, exe_wrap)
+@@ -822,7 +822,7 @@
+                 compiler_type = self.get_gnu_compiler_type(defines)
+                 version = self.get_gnu_version_from_defines(defines)
+                 return GnuObjCPPCompiler(ccache + compiler, version, compiler_type, is_cross, exe_wrap, defines)
+-            if out.startswith('Apple LLVM'):
++            if out.startswith('Apple LLVM') or mesonlib.for_darwin(want_cross, self):
+                 return ClangObjCPPCompiler(ccache + compiler, version, CompilerType.CLANG_OSX, is_cross, exe_wrap)
+             if out.startswith('clang'):
+                 return ClangObjCPPCompiler(ccache + compiler, version, CompilerType.CLANG_STANDARD, is_cross, exe_wrap)