lang/ecl: Add SAGE option

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.
• 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.

Submitted by: thierry (based on)
Reviewed by: thierry (original submitter)
Approved by: olgeni (maintainer), gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D25096