Add a configure option to enable LTO

Add a configure option to enable LTO

The supported options are:
- thin: this is the recommended option for clang, as it is more
  scalable. It automatically determines the number of CPU cores
  to use. It requires support in the linker as well.
- auto: this is the recommended option for GCC, and a valid
  fallback for clang when 'thin' is not supported. It automatically
  determines the number of CPU cores to use as well.
- yes: this translates to '-flto', which will use a single CPU core
  in most cases, but is a valid fallback.
- no: LTO is disabled.