Batch of fixes/changes slated for 2.4.1 release

- Added support for Clang/Clang++. Clang is automatically used if detected. Should work with Clang 14+

- Fixed examples/httpd: main() missing type specifier (should have been int, none present)

- Removed the need for the environment variable OSIRISDIR in the configure script, making it easier to use

- Removed configs for long obsolete platforms and platforms that have no planned support in Osiris.

- Bumped OSIRIS_VER and OSIRIS_VER_STR to reflect new Osiris release 2.4.1

- Fixed a bug in qtextbrowser that would cause a segfault when forwardStack was empty. This bug was introduced in Qt 2.1.0, with a release date of April 2000. That's a bugfix almost 23 years later!

- Modified qclipboard_x11 to enable QT_CLIPBOARD_DEBUG macro when configure is run with -debug. This provides some useful output when debugging clipboard issues and removes the need to manually edit a file to uncomment a macro.

- Added missing header file check in configure for libjpeg

- Fixed the libXft header file check in configure

- Fixed a minor spelling issue for a warning in designer/pixmapchooser

- Workaround for Designer to fix an issue where it washes out the window when the Composite extension is enabled on X11 displays

- libqt(,mt) is now libosiris(,mt)

- Updated configure script to clarify some printed license information

- Removed all other currently unsupported platform configs

- Added missing header file check in configure for Mesa libGLU

- Remove registration link and other obsolete items from Designer

- Update "About Qt" to "About Osiris" in Designer