cmake: make glut a soft dependency

There's parts of the tree that is useful even without GLUT, so let's allow building those without requiring GLUT. But let's emit a warning so users have a chance to understand why they get such a stripped-down build.

This is especially useful on Windows, where GLUT is a bit of an exotic dependency.

This gets us a bit closer to what we're doing with autotools.

src/CMakeLists.txt

 add_subdirectory (glad)
 
-add_subdirectory (util)
-
-add_subdirectory (demos)
-add_subdirectory (tests)
-add_subdirectory (trivial)
-add_subdirectory (redbook)
-add_subdirectory (samples)
-add_subdirectory (perf)
-add_subdirectory (objviewer)
-
-add_subdirectory (glsl)
-add_subdirectory (fp)
-add_subdirectory (fpglsl)
-add_subdirectory (vp)
-add_subdirectory (vpglsl)
-add_subdirectory (gs)
-add_subdirectory (tools)
+if (GLUT_FOUND)
+	add_subdirectory (util)
+
+	add_subdirectory (demos)
+	add_subdirectory (tests)
+	add_subdirectory (trivial)
+	add_subdirectory (redbook)
+	add_subdirectory (samples)
+	add_subdirectory (perf)
+	add_subdirectory (objviewer)
+
+	add_subdirectory (glsl)
+	add_subdirectory (fp)
+	add_subdirectory (fpglsl)
+	add_subdirectory (vp)
+	add_subdirectory (vpglsl)
+	add_subdirectory (gs)
+	add_subdirectory (tools)
+endif ()
 
 if (EGL_FOUND)
 	add_subdirectory(egl)