Add initial Abamake platorm support for FreeBSD
Change comment into brief attribute for the <abamk> element
Change <abamk*> elements into <abamake*>
Change abamake.job.Runner.running_jobs_max into getter/setter pair
Generalize Clang version detection to include “clang”, “FreeBSD clang” and “Apple LLVM”
Rename ABC_HOST_API_* to ABC_TARGET_API_*

They’re not about the host, but about the target.
Remove unnecessary #include
Initial support for compiling under *BSD
ENODATA is not defined under FreeBSD
Address Clang integer truncation warning
Link FreeBSD binaries against pthread
Split src/abaclade/exception-os.cxx by platform
Add ABC_TARGET_API_MACH for Mach kernel support
Further break up src/abaclade/exception-posix.cxx
Rename files
Rewrite POSIX signal handler for hard errors to emulate call at fault site
Add missing #include
Provide access into ucontext_t for FreeBSD
Rename ABC_TARGET_API_BSD to ABC_TARGET_API_FREEBSD

It’s only used to enable FreeBSD-only code paths.
Rename ABC_TARGET_API_* back to ABC_HOST_API_*

After all, it makes more sense.