libmodplug: Fix build on darwin

* removing the `-mmacosx-version-min` flag makes the build work again
* also fix the path to the `file` command in the configure script
  to fix the warning

```
./configure: line 7754: /usr/bin/file: No such file or directory
```

pkgs/development/libraries/libmodplug/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl }:
+{ stdenv, fetchurl, file }:
 
 let
   version = "0.8.9.0";
@@ -6,6 +6,12 @@ in stdenv.mkDerivation rec {
   pname = "libmodplug";
   inherit version;
 
+  preConfigure = ''
+     substituteInPlace configure \
+        --replace ' -mmacosx-version-min=10.5' "" \
+        --replace /usr/bin/file ${file}/bin/file
+  '';
+
   meta = with stdenv.lib; {
     description = "MOD playing library";
     homepage    = "http://modplug-xmms.sourceforge.net/";