Force the iOS release flags into the cache

The iOS toolchain listed a nonexistent variable instead of the release
flags it modifies, so try_compile checks did not use these flags.

Name the release flags variable.

Change-Id: Iad39e083e260d576c4cd1327bd77c364ddba8592

cmake/iOS.cmake

diff --git a/cmake/iOS.cmake b/cmake/iOS.cmake
index 805ce3e..6be1acd 100644
--- a/cmake/iOS.cmake
+++ b/cmake/iOS.cmake
@@ -272,7 +272,7 @@
 list(APPEND VARS_TO_FORCE_IN_CACHE
   CMAKE_C_FLAGS
   CMAKE_CXX_FLAGS
-  CMAKE_CXX_RELEASE
+  CMAKE_CXX_FLAGS_RELEASE
   CMAKE_C_LINK_FLAGS
   CMAKE_CXX_LINK_FLAGS)
 foreach(VAR_TO_FORCE ${VARS_TO_FORCE_IN_CACHE})