User setting to turn LTO on and off

This isn't currently working on mingw

CMakeLists.txt

@@ -92,11 +92,6 @@ else()
   else()
     set(WARNINGS "${WARNINGS} -Wlogical-op -Wno-error=maybe-uninitialized")
   endif()
-  # Since gcc 4.9 the LTO format is non-standard (slim), so we need the gcc-specific ar and ranlib binaries
-  if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9.0))
-    set(CMAKE_AR "gcc-ar")
-    set(CMAKE_RANLIB "gcc-ranlib")
-  endif()
   if(MINGW)
     set(WARNINGS "${WARNINGS} -Wno-error=unused-value -Wno-error=unused-but-set-variable")
     set(MINGW_FLAG "${MINGW_FLAG} -DWIN32_LEAN_AND_MEAN")
@@ -121,14 +116,23 @@ else()
   else()
     set(DEBUG_FLAGS "-g3 -O0")
   endif()
+
   set(RELEASE_FLAGS "-Ofast -DNDEBUG -Wno-unused-variable")
-  if(NOT CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+
+  set(USE_LTO false CACHE BOOL "Use Link-Time Optimization (Release mode only)")
+  if(USE_LTO)
     # There is a clang bug that does not allow to compile code that uses AES-NI intrinsics if -flto is enabled
     set(RELEASE_FLAGS "${RELEASE_FLAGS} -flto")
+    if(STATIC)
+      set(RELEASE_FLAGS "${RELEASE_FLAGS} -ffat-lto-objects")
+    endif()
+    # Since gcc 4.9 the LTO format is non-standard (slim), so we need the gcc-specific ar and ranlib binaries
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9.0))
+      set(CMAKE_AR "gcc-ar")
+      set(CMAKE_RANLIB "gcc-ranlib")
+    endif()
   endif()
-  #if(CMAKE_C_COMPILER_ID STREQUAL "GNU" AND NOT MINGW)
-  #  set(RELEASE_FLAGS "${RELEASE_FLAGS} -fno-fat-lto-objects")
-  #endif()
+
   set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} ${DEBUG_FLAGS}")
   set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} ${DEBUG_FLAGS}")
   set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE} ${RELEASE_FLAGS}")