microsoft_gsl: fix crossSystem build support

cmake needs to be part of nativeBuildInputs, otherwise you get a very unhelpful
debugging trace.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

pkgs/development/libraries/microsoft_gsl/default.nix

@@ -1,5 +1,10 @@
-{ stdenv, fetchgit, cmake }:
+{ stdenv, fetchgit, cmake
+, hostPlatform, buildPlatform
+}:
 
+let
+  nativeBuild = hostPlatform == buildPlatform;
+in
 stdenv.mkDerivation rec {
   name = "microsoft_gsl-${version}";
   version = "2017-02-13";
@@ -10,8 +15,11 @@ stdenv.mkDerivation rec {
     sha256 = "03d17mnx6n175aakin313308q14wzvaa9pd0m1yfk6ckhha4qf35";
   };
 
-  # build phase just runs the unit tests
-  buildInputs = [ cmake ];
+
+  # build phase just runs the unit tests, so skip it if
+  # we're doing a cross build
+  nativeBuildInputs = [ cmake ];
+  buildPhase = if nativeBuild then "make" else "true";
 
   installPhase = ''
     mkdir -p $out/include
@@ -20,9 +28,9 @@ stdenv.mkDerivation rec {
 
   meta = with stdenv.lib; {
     description = "Functions and types that are suggested for use by the C++ Core Guidelines";
-    homepage = https://github.com/Microsoft/GSL;
-    license = licenses.mit;
-    platforms = platforms.all;
-    maintainers = with maintainers; [ xwvvvvwx ];
+    homepage    = https://github.com/Microsoft/GSL;
+    license     = licenses.mit;
+    platforms   = platforms.all;
+    maintainers = with maintainers; [ thoughtpolice xwvvvvwx ];
   };
 }