Enabling -O4 (link-time optimization) only if compiler/linker support it.

- The -O4 option requires the linker to have bitcode support, currently for
  clang this means using the gold linker and the LLVM-gold plugin:
  http://llvm.org/docs/GoldPlugin.html.
- Otherwise you get (confusing) 'file format not recognised' errors ala:
  http://llvm.org/bugs/show_bug.cgi?id=9897.
- Adding explicit check for LTO support as at least some package installs of
  clang on linux do not use the gold linker by default.

Change-Id: I2a4c670e470d9b48da2a15b7e91a59fb4ad3e8ad

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e3fcd19..c3b536b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -614,7 +614,17 @@
     ENDIF (${BUILD_ANDROID})
   ENDIF (CMAKE_COMPILER_IS_GNUCXX)
   IF ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
-    SET(CERES_CXX_FLAGS "${CERES_CXX_FLAGS} -O4")
+    # Use of -O4 requires use of gold linker & LLVM-gold plugin, which might 
+    # well not be present / in use and without which files will compile, but 
+    # not link ('file not recognized') so explicitly check for support
+    INCLUDE(CheckCXXCompilerFlag)
+    CHECK_CXX_COMPILER_FLAG("-O4" HAVE_LTO_SUPPORT)
+    IF (HAVE_LTO_SUPPORT)
+      MESSAGE(STATUS "Enabling link-time optimization (-O4)")
+      SET(CERES_CXX_FLAGS "${CERES_CXX_FLAGS} -O4")
+    ELSE()
+      MESSAGE(STATUS "Compiler/linker does not support link-time optimization (-O4), disabling.")
+    ENDIF (HAVE_LTO_SUPPORT)
   ENDIF()
 ENDIF (CMAKE_BUILD_TYPE STREQUAL "Release")