[freeglut] Update to 3.4.0, cleanup

[dg0yt]

What does your PR fix?

Updates freeglut.
Restores upstream's X11 dependency handling with variables instead of targets, fixing the exported FreeGLUT config.
Adds missing X11 lib usage requirement to wrapper. (Discovered while working on #28311.)

Which triplets are supported/not supported? Have you updated the CI baseline?

unchanged

Resolving cross-platform file name inconsistencies

FreeGLUT's default configuration comes with these filename inconsistencies:

• Only Windows (but not Mingw) applies a d debug suffix to the lib name.
• Windows (incl. Mingw) uses the freeglut name for shared libs and the freeglut_static name for static libs.
• Non-Windows uses the glut name for libs.
• The lib name is also used for the pc file, so it varies as well (freeglut.pc vs. glut.pc).
• Basically, the different names are following different defaults of the FREEGLUT_REPLACE_GLUT options. This options also controls whether FreeGLUT installs include/GL/glut.h.

The former patching in vcpkg removed the _static lib name component.
These library names are kept for compatibility with downstream find_library.
But apart from the lib names, this PR now always sets FREEGLUT_REPLACE_GLUT to on.

• No changes to the previous lib names (by patching).
• include/GL/glut.h is always installed.
• The pc file is always installed as glut.pc.
  Benefit: This is the first pkg-config file looked up by FindGLUT.cmake.
• A copy of this pc file is created as freeglut.pc.
  So both names are always available.

[vicroms]

Thanks for the detailed description! It helped a lot during review.