dmd: Avoid depending on gcc for the darwin build (close #11949)

Adding stdenv.cc into the PATH, also setting CC, so that on Darwin
clang will be used by default. Still allowing to use an existing value
of CC if it is set already.

Replacing __inline_isnanl with __inline_isnan on darwin since the former
one was not defined.

pkgs/development/compilers/dmd/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, unzip, curl, makeWrapper, gcc }:
+{ stdenv, fetchurl, unzip, curl, makeWrapper }:
 
 stdenv.mkDerivation {
   name = "dmd-2.067.1";
@@ -10,9 +10,15 @@ stdenv.mkDerivation {
 
   buildInputs = [ unzip curl makeWrapper ];
 
-  # Allow to use "clang++", commented in Makefile
   postPatch = stdenv.lib.optionalString stdenv.isDarwin ''
-      substituteInPlace src/dmd/posix.mak --replace g++ clang++
+      # Allow to use "clang++", commented in Makefile
+      substituteInPlace src/dmd/posix.mak \
+          --replace g++ clang++ \
+          --replace MACOSX_DEPLOYMENT_TARGET MACOSX_DEPLOYMENT_TARGET_
+
+      # Was not able to compile on darwin due to "__inline_isnanl"
+      # being undefined.
+      substituteInPlace src/dmd/root/port.c --replace __inline_isnanl __inline_isnan
   '';
 
   # Buid and install are based on http://wiki.dlang.org/Building_DMD
@@ -48,7 +54,9 @@ stdenv.mkDerivation {
       cp -r std $out/include/d2
       cp -r etc $out/include/d2
 
-      wrapProgram $out/bin/dmd --prefix PATH ":" "${gcc}/bin/"
+      wrapProgram $out/bin/dmd \
+          --prefix PATH ":" "${stdenv.cc}/bin" \
+          --set CC "$""{CC:-$CC""}"
 
       cd $out/bin
       tee dmd.conf << EOF