don't do -ipo check for non-intel compilers (causes issues with Clang on OSX)

CMakeLists.txt

@@ -64,11 +64,13 @@ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU"
     endif()
 
     # Intel equivalent to LTO is called IPO
+    if (CMAKE_CXX_COMPILER_ID MATCHES "Intel")
       CHECK_CXX_COMPILER_FLAG("-ipo" HAS_IPO_FLAG)
       if (HAS_IPO_FLAG)
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -ipo")
       endif()
     endif()
+  endif()
 endif()
 
 # Compile with compiler warnings turned on