FindOpenGL: Drop Cygwin-specific behavior and use POSIX code path

Based on downstream patch from Cygwin package for CMake by Marco Atzeri:

* https://github.com/matzeri/cygwin-pkg/blob/64864eb8f0d635ea44226e4b9a41a7ca59a1c7dd/cmake/3.17.3-opengl.patch

Modules/FindOpenGL.cmake

@@ -132,19 +132,7 @@ endforeach()
 
 set(_OpenGL_CACHE_VARS)
 
-if (CYGWIN)
-  find_path(OPENGL_INCLUDE_DIR GL/gl.h )
-  list(APPEND _OpenGL_REQUIRED_VARS OPENGL_INCLUDE_DIR)
-
-  find_library(OPENGL_gl_LIBRARY opengl32 )
-  find_library(OPENGL_glu_LIBRARY glu32 )
-
-  list(APPEND _OpenGL_CACHE_VARS
-    OPENGL_INCLUDE_DIR
-    OPENGL_gl_LIBRARY
-    OPENGL_glu_LIBRARY
-    )
-elseif (WIN32)
+if (WIN32)
 
   if(BORLAND)
     set (OPENGL_gl_LIBRARY import32 CACHE STRING "OpenGL library for win32")