Introduce a new option called SAGE and enabled by default in order to have ecl working with sage.
Also:
- Move bohem-gc dependency from THREADS_LIB_DEPENDS_OFF to LIB_DEPENDS.
- Add USES+= autoreconf localbase
- Remove CFLAGS and LDFLAGS.
This work is based on https://reviews.freebsd.org/D24958 submitted by thierry@.