devel/hs-gtk2hs-buildtools: Fix build of www/hs-gtk with gcc5

The Haskell gtk buildtools use CPP. The CPP on gcc5 has changed
behavior, thus preventing hs-gtk from successfully building with gcc5.
By adding the -P argument when invoking "cpp" which forces the old
behavior, hs-gtk can be built by all versions of gcc.

PR: 197924
Submitted by: marino
Approved by: haskell@ (pgj@)