ghcHEAD: Force INTEGER_LIBRARY like the other GHCs

We really need to abstract to avoid these copy-paste errors.

pkgs/development/compilers/ghc/head.nix

@@ -32,6 +32,8 @@
   ghcFlavour ? stdenv.lib.optionalString (stdenv.targetPlatform != stdenv.hostPlatform) "perf-cross"
 }:
 
+assert !enableIntegerSimple -> gmp != null;
+
 let
   inherit (stdenv) buildPlatform hostPlatform targetPlatform;
 
@@ -48,8 +50,7 @@ let
     include mk/flavours/\$(BuildFlavour).mk
     endif
     DYNAMIC_GHC_PROGRAMS = ${if enableShared then "YES" else "NO"}
-  '' + stdenv.lib.optionalString enableIntegerSimple ''
-    INTEGER_LIBRARY = integer-simple
+    INTEGER_LIBRARY = ${if enableIntegerSimple then "integer-simple" else "integer-gmp"}
   '' + stdenv.lib.optionalString (targetPlatform != hostPlatform) ''
     Stage1Only = ${if targetPlatform.system == hostPlatform.system then "NO" else "YES"}
     CrossCompilePrefix = ${targetPrefix}