Fix "fully support ... own shared libraries" (#1238)

- If shared libraries are built, then fluid, fltk-options, and the
  "games" are linked against the shared FLTK libraries. On some
  platforms the static and the shared versions of fluid and
  fltk-options are built. The games are only built if
  FLTK_BUILD_TEST is enabled.

- The CMake 'install' target now installs the games (if built)
  and their man pages on all platforms (no matter if that is
  useful, for instance on Windows).

- On macOS 'CMAKE_INSTALL_RPATH' is set so *installed* programs
  automatically find their shared FLTK libraries. The "shared"
  versions of fluid and fltk-options got their own '.plist' files.
  This works for both the executables themselves as well as those
  included in bundles. There may be more to do on the macOS platform.

.github/workflows/build.yml

@@ -103,7 +103,7 @@ jobs:
       # Note the current convention is to use the -S and -B options here to specify source
       # and build directories, but this is only available with CMake 3.13 and higher.
       # The CMake binaries on the Github Actions machines are (as of this writing) 3.12
-      run: cmake $GITHUB_WORKSPACE -D CMAKE_BUILD_TYPE=$BUILD_TYPE -D CMAKE_CXX_STANDARD=11 -D CMAKE_CXX_EXTENSIONS=OFF -D CMAKE_C_FLAGS_INIT="-Wall -Wunused" -D CMAKE_CXX_FLAGS_INIT="-Wall -Wunused -Wsuggest-override"
+      run: cmake $GITHUB_WORKSPACE -D CMAKE_BUILD_TYPE=$BUILD_TYPE -DFLTK_BUILD_SHARED_LIBS:BOOL=ON -D CMAKE_CXX_STANDARD=11 -D CMAKE_CXX_EXTENSIONS=OFF -D CMAKE_C_FLAGS_INIT="-Wall -Wunused" -D CMAKE_CXX_FLAGS_INIT="-Wall -Wunused -Wsuggest-override"
 
     - name: Build
       working-directory: ${{github.workspace}}/build