Fix idrisPackages.with-packages

1. The ln step was failing due to a file already existing
2. gcc was invoked directly which caused failure on OS X.

pkgs/development/idris-modules/with-packages.nix

@@ -10,7 +10,7 @@
 
     installIdrisLib () {
       if [ -d $1/lib/${idris.name} ]; then
-        ln -sv $1/lib/${idris.name}/* $out/lib/${idris.name}
+        ln -fsv $1/lib/${idris.name}/* $out/lib/${idris.name}
       fi
     }
 
@@ -34,7 +34,7 @@
   '';
 
   buildPhase = ''
-    gcc -O3 -o idris idris.c
+    $CC -O3 -o idris idris.c
   '';
 
   installPhase = ''