libdecor/build/Makefile: unused anyway (previously used building demos)
src/CMakeLists.txt: broke some user builds, see fltk.general, thread
"-no-pie in FLTK Libraries build." (Mar 17, 2024).
Todo: check usage of "-fPIC" (libdecor builds)
This commit makes the default FLTK build setting use libdecor
as packaged in Linux when the build system contains packages
libdecor-0-dev and libdecor-0-plugin-1-gtk in version ≥ 0.2.0.
Otherwise, FLTK uses the bundled version of libdecor.
This includes situations where package libdecor-0-dev is
present in an earlier version.
1) We don't need to build the libdecor demo programs for FLTK.
2) The Wayland build process generates some source and header files
in the main src/ directory. These files are now ignored by git.
