Simplify cross-building with Fortran support

I am cross-building HDF5 2.0.0 for various architectures with support for the Fortran API enabled. This is very tedious.

The cmake build system detects various properties (such as `sizeof(int)`) for the C language, but does not do so for the Fortran language. cmake needs to run a Fortran program for this, and this doesn't work when cross-building. The only way to make this work is to provide the respective output (which would have been produced by the Fortran program) to cmake. This is very tedious.

My feature request is to rewrite the cmake build system for Fortran so that it can determine the Fortran language properties without running any executables, in the same way it can do so for C.