Set both TARGET_NACL and MAGNUM_TARGET_NACL in CMakeLists.

Being consistent with all other options, all of them are both
unprexfixed and prefixed in CMake (and then only prefixed versions are
saved into `magnumConfigure.h` file).

CMakeLists.txt

@@ -68,6 +68,7 @@ set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/modules/")
 if(${CMAKE_SYSTEM_NAME} STREQUAL NaCl)
     set(TARGET_GLES 1)
     set(TARGET_GLES2 1)
+    set(TARGET_NACL 1)
     set(MAGNUM_TARGET_NACL 1)
 endif()