external: install gl2ext.h also for ES3.

Again, didn't bother with testing.

external/OpenGL/CMakeLists.txt

@@ -26,9 +26,8 @@ if(NOT TARGET_GLES)
     add_subdirectory(GL)
 else()
     add_subdirectory(KHR)
-    if(TARGET_GLES2)
-        add_subdirectory(GLES2)
-    else()
+    add_subdirectory(GLES2)
+    if(NOT TARGET_GLES2)
         add_subdirectory(GLES3)
     endif()
 endif()

external/OpenGL/GLES2/CMakeLists.txt

@@ -25,8 +25,9 @@
 set(MagnumOpenGL_HEADERS
     gl2ext.h)
 
-# NaCl has its own gl2.h, this one causes linker issues
-if(NOT CORRADE_TARGET_NACL)
+# We need gl2ext.h also for ES3 (but not the others). NaCl has its own gl2.h,
+# this one causes linker issues
+if(TARGET_GLES2 AND NOT CORRADE_TARGET_NACL)
     set(MagnumOpenGL_HEADERS ${MagnumOpenGL_HEADERS}
         gl2platform.h
         gl2.h)