[CMake] Mixing C++ and Fortran

[Honest Guvnor]

I am building a program with a C++ main (WxWidgets) and linking with C
and Fortran libraries. My problem is that on each platform I am
currently having to find the basic Fortran library (e.g. libg2c for
g77) and include it explicitly in the target_link_libraries list.

I have tried adding a dummy fortran file to the list of C++ files to
force it know about Fortran and get prompted to declare the
linker_language property. Setting this to C++ results in no change. I
have downloaded and installed the current version 2.4.8 with no
change. I have tried OSX and Linux with the same result. I have mixed
Fortran and C in another program without problems. A mix of C++ and C
was working in this program until the addition of the Fortran
libraries.

Suggestions gratefully received.