Gcc 4.9 LTO fix

The new lto format requires use of the gcc-provided gcc-ar and
gcc-ranlib binaries.

CMakeLists.txt

@@ -44,6 +44,11 @@ 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")
     set(MINGW_FLAG "-DWIN32_LEAN_AND_MEAN")