Add support for thin LTO

The b_lto option has been updated and now can be set to the value thin. This enables thin LTO on all compilers where it is supported. At the time of writing this means only Clang.

This change is potentially backwards incompatible. If you have examined the value of b_lto in your build file, note that its type has changed from a boolean to a string. Thus comparisons like this:

if get_option('b_lto')
...
endif

need to be changed to something like this instead:

if get_option('b_lto') == 'true'
...
endif

This should not affect any comman line invocations as configuring LTO with -Db_lto=true still works and behaves the same way as before.