Introduce a new option called SAGE and enabled by default in order to have ecl working with sage.
- Move bohem-gc dependency from THREADS_LIB_DEPENDS_OFF to LIB_DEPENDS.
- Build with libatomic_ops from ports rather than with the bundled library.
- Replace explicit settings of CFLAGS and LDFLAGS by USES=localbase.
- Add USES+= autoreconf.
This work is based on https://reviews.freebsd.org/D24958 submitted by thierry@.