cmake: Add a flag to enable LTO

This adds -DLLVM_ENABLE_LTO, rather than forcing people to manually
add -flto to the various _FLAGS variables.

llvm-svn: 259766
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 9b6a590..99b2318 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -656,6 +656,13 @@
   CMAKE_EXE_LINKER_FLAGS
   CMAKE_SHARED_LINKER_FLAGS)
 
+option(LLVM_ENABLE_LTO "Enable link-time optimization" OFF)
+append_if(LLVM_ENABLE_LTO "-flto"
+  CMAKE_CXX_FLAGS
+  CMAKE_C_FLAGS
+  CMAKE_EXE_LINKER_FLAGS
+  CMAKE_SHARED_LINKER_FLAGS)
+
 # Plugin support
 # FIXME: Make this configurable.
 if(WIN32 OR CYGWIN)