tree b55683ec787ba1a68a19fef4b17b6959553796cb
parent a6b76c7ff67d84ae89271f8164a55ac5b4db3e7a
author Alex Stewart <alexs.mac@gmail.com> 1368385564 +0100
committer Alex Stewart <alexs.mac@gmail.com> 1368463135 +0100

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