FindOpenGL: Revert support for imported targets (#15267)

Revert the feature added by commit v3.1.0-rc1~420^2~2 (FindOpenGL:
Provide imported targets for GL and GLU, 2014-05-31).  Unfortunately it
does not work on Windows because the full path to each library file is
not actually known.  The IMPORTED_LOCATION of an imported target must be
a full path, but OPENGL_gl_LIBRARY is just 'opengl32' on Windows because
the actual library file is in some implicit link directory that we may
know know.

More infrastructure will be needed in CMake to allow a name-only
imported library.  Until that exists, we will not be able to provide
imported targets in FindOpenGL.

Help/release/3.1.0.rst

@@ -230,9 +230,6 @@ Modules
 
 * The :module:`FindOpenCL` module was introduced.
 
-* The :module:`FindOpenGL` module now provides imported targets
-  ``OpenGL::GL`` and ``OpenGL::GLU`` when the libraries are found.
-
 * The :module:`FindOpenMP` module learned to support Fortran.
 
 * The :module:`FindPkgConfig` module learned to use the ``PKG_CONFIG``

Modules/FindOpenGL.cmake

@@ -4,16 +4,6 @@
 #
 # FindModule for OpenGL and GLU.
 #
-# IMPORTED Targets
-# ^^^^^^^^^^^^^^^^
-#
-# This module defines the :prop_tgt:`IMPORTED` targets:
-#
-# ``OpenGL::GL``
-#  Defined if the system has OpenGL.
-# ``OpenGL::GLU``
-#  Defined if the system has GLU.
-#
 # Result Variables
 # ^^^^^^^^^^^^^^^^
 #
@@ -191,36 +181,6 @@ include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(OpenGL REQUIRED_VARS ${_OpenGL_REQUIRED_VARS})
 unset(_OpenGL_REQUIRED_VARS)
 
-# OpenGL:: targets
-if(OPENGL_FOUND)
-  if(NOT TARGET OpenGL::GL)
-    add_library(OpenGL::GL UNKNOWN IMPORTED)
-    set_target_properties(OpenGL::GL PROPERTIES
-      INTERFACE_INCLUDE_DIRECTORIES "${OPENGL_INCLUDE_DIR}")
-    if(OPENGL_gl_LIBRARY MATCHES "/([^/]+)\\.framework$")
-      set_target_properties(OpenGL::GL PROPERTIES
-        IMPORTED_LOCATION "${OPENGL_gl_LIBRARY}/${CMAKE_MATCH_1}")
-    else()
-      set_target_properties(OpenGL::GL PROPERTIES
-        IMPORTED_LOCATION "${OPENGL_gl_LIBRARY}")
-    endif()
-  endif()
-
-  if(OPENGL_GLU_FOUND AND NOT TARGET OpenGL::GLU)
-    add_library(OpenGL::GLU UNKNOWN IMPORTED)
-    set_target_properties(OpenGL::GLU PROPERTIES
-      INTERFACE_INCLUDE_DIRECTORIES "${OPENGL_INCLUDE_DIR}"
-      INTERFACE_LINK_LIBRARIES OpenGL::GL)
-    if(OPENGL_glu_LIBRARY MATCHES "/([^/]+)\\.framework$")
-      set_target_properties(OpenGL::GLU PROPERTIES
-        IMPORTED_LOCATION "${OPENGL_glu_LIBRARY}/${CMAKE_MATCH_1}")
-    else()
-      set_target_properties(OpenGL::GLU PROPERTIES
-        IMPORTED_LOCATION "${OPENGL_glu_LIBRARY}")
-    endif()
-  endif()
-endif()
-
 mark_as_advanced(
   OPENGL_INCLUDE_DIR
   OPENGL_xmesa_INCLUDE_DIR