Restore macOS GLU detection and test runtime loading

[tritao]

This fixes a regression in the latest FreeCAD macOS weekly build, identified and debugged by @maxwxyz.

The Coin update included by FreeCAD commit 521f19d16d incorporated Coin commit 5e40f18505e8, which accidentally removed the macOS GLU header check:

check_include_file(OpenGL/glu.h HAVE_OPENGL_GLU_H)

Clean macOS configurations therefore left HAVE_OPENGL_GLU_H undefined.

With GLU_RUNTIME_LINKING and GLU_IS_PART_OF_GL enabled, this prevented GLUWrapper from opening the current process image and resolving GLU symbols from Apple’s OpenGL framework. Optimized Release builds reduced this path to available = 0, disabling GLU-dependent functionality without producing a build or linker error.

This ended up causing issues in the font loading backends, which need GLU for font rasterization.

Existing CI did not catch the regression because Coin still compiled successfully and no test verified that the GLU runtime loader was actually usable.

Solution

Restore the OpenGL/glu.h probe for native macOS builds:

if(APPLE AND NOT COIN_BUILD_MAC_X11)
  check_include_file(OpenGL/glu.h HAVE_OPENGL_GLU_H)
  check_include_file(OpenGL/CGLCurrent.h HAVE_OPENGL_CGLCURRENT_H)
endif()

Add a headless runtime test that verifies configurations where GLU is part of the linked OpenGL framework can:

• initialize GLUWrapper;
• report GLU as available;
• resolve the GLU tessellator functions;
• create and destroy a GLU tessellator.

The private-wrapper test runs on macOS and Linux. It is excluded on Windows because GLUWrapper is intentionally not exported from the Coin DLL.

[tritao]

Tested by @maxwxyz