mpd: fix build on x86_64-darwin

The darwin-specific AssertMacros header has some legacy macros with very
generic names such as "check" which happen to conflict with mpd's
headers. We can explicitly opt out of these macros - at some point in
the future, they will not be defined by default.

Also remove stdenv override, not needed now that x86_64-darwin uses LLVM 11

pkgs/servers/mpd/default.nix

@@ -157,6 +157,10 @@ let
       outputs = [ "out" "doc" ]
         ++ lib.optional (builtins.elem "documentation" features_) "man";
 
+      CXXFLAGS = lib.optionals stdenv.isDarwin [
+        "-D__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORES=0"
+      ];
+
       mesonFlags = [
         "-Dtest=true"
         "-Dmanpages=true"

pkgs/top-level/all-packages.nix

@@ -21291,7 +21291,6 @@ with pkgs;
   mod_timestamp = apacheHttpdPackages.mod_timestamp;
 
   inherit (callPackages ../servers/mpd {
-    stdenv = if (with stdenv; cc.isClang && isx86_64) then llvmPackages_8.stdenv else stdenv;
     inherit (darwin.apple_sdk.frameworks) AudioToolbox AudioUnit;
   }) mpd mpd-small mpdWithFeatures;