Allow using cross-compilers to bootstrap GHC.

In my case I'm using "cross"-compiler from
x86_64-unknown-linux to x86_64-HEAD-linux.

Those targets have the same ABI and can boot one another.
--- a/configure.ac
+++ b/configure.ac
@@ -219,7 +219,7 @@ then
     echo "This GHC (${WithGhc}) does not generate code for the build platform"
     echo "   GHC target platform    : $bootstrap_target"
     echo "   Desired build platform : $BuildPlatform"
-    exit 1
+    #exit 1
 fi
 
 # Testing if we shall enable shared libs support on Solaris.