CMake: avoid confusion about two different fltk-config files in build tree

[Albrecht-S]

There has been confusion about two different fltk-config files being created in the CMake build tree. See this reply in an old thread in fltk.coredev ("Regression in CMake-based build"). Note that the mentioned regression has been fixed and has nothing to do with this issue here.

The reply mentions details which might be required for different build types, including multi-config builds like Xcode and Visual Studio.

This issue is a reminder to check this old thread and move the generated fltk-config files to appropriate places and/or generate the fltk-config file that is to be installed during the installation process if possible.

[Albrecht-S]

This issue will not be "fixed" in FLTK 1.4.x, I'll take care of this in 1.5.0 to avoid even more confusion for users already used to the way it is in 1.4.0 (Git branch master) as of today.