eina_btlog: add Mac OS X support for backtrace

eina_btlog: add Mac OS X support for backtrace

This was actually difficult...
Mac OS X can use addr2line (sometimes called gaddr2line in
function of the package managers). However, addr2line does
NOT handle specific cases.

It was therefore necessary to use Mac OS X' own tool: atos,
which gracefully handles all backtraces, including the one
containing objective-c messages or fat archives.

eina_btlog now tests different utilities one by one, and
determines whether it is supported or not.

Fixes T3711