Fix modern Automake's warning about subdir-objects.
Ignore admin/compile added by new Automake.
Fix bootstrap script POSIX compliance.

The OSTYPE variable is a bash-ism, e.g.  dash doesn't define it.  Fix
the code to work with any POSIX compliant shell.

Fixes #17.
Don't require doxygen & bakefile to bootstrap.

These tools are only used for Windows (bkl) or non-essential stuff, so
it's safe to skip them on platforms where ./boostrap would typically be
used.

See #17.