Simplify logic to compute install prefix in OS X CMake.app

Also avoid explicitly dereferencing a variable in an if() condition.

CMakeLists.txt

@@ -543,10 +543,7 @@ if(NOT CMake_TEST_EXTERNAL_CMAKE)
         "${CMake_VERSION_MAJOR}.${CMake_VERSION_MINOR}.${CMake_VERSION_PATCH}")
       set(CMAKE_BUNDLE_LOCATION "${CMAKE_INSTALL_PREFIX}")
       # make sure CMAKE_INSTALL_PREFIX ends in /
-      string(LENGTH "${CMAKE_INSTALL_PREFIX}" LEN)
-      math(EXPR LEN "${LEN} -1" )
-      string(SUBSTRING "${CMAKE_INSTALL_PREFIX}" ${LEN} 1 ENDCH)
-      if(NOT "${ENDCH}" STREQUAL "/")
+      if(NOT CMAKE_INSTALL_PREFIX MATCHES "/$")
         set(CMAKE_INSTALL_PREFIX "${CMAKE_INSTALL_PREFIX}/")
       endif()
       set(CMAKE_INSTALL_PREFIX