Workaround for non-working fltk-config
fltk-config is a bash script and on windows, if there's no bash environment FLTK_VERSION will be empty and result in a CMake error. This fixes it by quoting the version
fltk-config is a bash script and on windows, if there's no bash environment FLTK_VERSION will be empty and result in a CMake error. This fixes it by quoting the version