CMake: don't expose TARGET_*GLES* when WITH_GL is OFF.

Also add a heads-up warning to FindMagnum.cmake about this.

CMakeLists.txt

@@ -221,6 +221,15 @@ if(WITH_GL)
     else()
         find_package(OpenGLES3 REQUIRED)
     endif()
+else()
+    if(TARGET_GL)
+        message(FATAL_ERROR "WITH_GL is OFF but TARGET_GL is enabled. Hmm.")
+    endif()
+    # These are implicitly enabled for embedded platforms, disable them all for
+    # consistency
+    set(TARGET_GLES OFF)
+    set(TARGET_GLES2 OFF)
+    set(TARGET_DESKTOP_GLES OFF)
 endif()
 
 # Configuration variables (saved later to configure.h)
@@ -234,13 +243,13 @@ if(TARGET_GLES)
     else()
         set(MAGNUM_TARGET_GLES3 1)
     endif()
-endif()
-if(CORRADE_TARGET_EMSCRIPTEN)
+    if(CORRADE_TARGET_EMSCRIPTEN)
         set(TARGET_WEBGL 1)
         set(MAGNUM_TARGET_WEBGL 1)
-endif()
-if(TARGET_DESKTOP_GLES)
+    endif()
+    if(TARGET_DESKTOP_GLES)
         set(MAGNUM_TARGET_DESKTOP_GLES 1)
+    endif()
 endif()
 if(TARGET_HEADLESS)
     set(MAGNUM_TARGET_HEADLESS 1)

modules/FindMagnum.cmake

@@ -241,6 +241,9 @@ endif()
 # Read flags from configuration
 file(READ ${_MAGNUM_CONFIGURE_FILE} _magnumConfigure)
 set(_magnumFlags
+    # WARNING: CAREFUL HERE, the string(FIND) succeeds even if a subset is
+    # found -- so e.g. looking for TARGET_GL will match TARGET_GLES2 as well.
+    # So far that's not a problem, but might become an issue for new flags.
     BUILD_DEPRECATED
     BUILD_STATIC
     BUILD_MULTITHREADED