In configure, test that GHC generates code for the correct platform (#4819)

In configure, test that GHC generates code for the correct platform (#4819)
Patch supplied by the bug reporter, tidied up by me.

$ ./configure --with-ghc=$HOME/fp/bin/i386-unknown-linux/ghc --build=x86_64-unknown-linux
checking for gfind... no
checking for find... /usr/bin/find
checking for sort... /usr/bin/sort
checking for GHC version date... inferred 7.1.20110107
checking version of ghc... 7.0.1
checking build system type... x86_64-unknown-linux-gnu
checking host system type... x86_64-unknown-linux-gnu
checking target system type... x86_64-unknown-linux-gnu
Host platform inferred as: i386-unknown-linux
Target platform inferred as: i386-unknown-linux
This GHC (/home/simonmar/fp/bin/i386-unknown-linux/ghc) does not generate code for the build platform
   GHC target platform    : i386-unknown-linux
   Desired build platform : x86_64-unknown-linux