Fortran: Add compiler ID/Version generator expressions

Adds `Fortran_COMPILER_ID` and `Fortran_COMPILER_VERSION` generator
expression support to match equivalent `C_COMPILER_ID`,
`CXX_COMPILER_ID`, `C_COMPILER_VERSION`, and `CXX_COMPILER_VERSION`
support.

This is very helpful in the case where the C/C++ compiler suite is a
different type of compiler from the platform Fortran compiler and
projects use generator expressions to assign compiler flags and
definitions.  (e.g. `GNU` C/C++ and `SunPro` Fortran on Linux)