This allows us to override the optimization level as not all platforms can
deal with -O3.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -847,6 +847,12 @@ if( MINGW AND NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
   llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELEASE "-O3" "-O2")
 endif()
 
+set(VOID_CXX_OPT_FLAGS "" CACHE STRING "Optimization level to use")
+
+if(NOT VOID_CXX_OPT_FLAGS STREQUAL "")
+  llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELEASE "-O3" "${VOID_CXX_OPT_FLAGS}")
+endif()
+
 # Put this before tblgen. Else we have a circular dependence.
 add_subdirectory(lib/Demangle)
 add_subdirectory(lib/Support)