lib/README.txt: clarify where built libraries are located
documentation/src/basics.dox: clarify (C++) compiler command usage
and improve documentation of the fltk-config script
Files without '.txt' suffix can't be handled well by Windows (users),
hence renaming such text files to *.txt makes FLTK more "Windows-friendly".
There are a few exceptions (this is intentional): README and other files
in bundled libraries (maintained upstream) are not changed.