Instrument and improve GC time

Instrument and improve GC time

Setting GC_free_space_divisor to 1 decreases GC time on GUILE v2 from
2.1s to 1.75s, a 17% speedup for the Mozart horn concerto example

Don't try to tune the GUILE 1.8 GC if we are on GUILE 2.

[hanwenn, 2020-01-24 10:03:51]

cc

[hanwenn, 2020-01-24 17:01:50]

not LGtM - the setting is marked deprecated

[hanwenn, 2020-01-24 17:21:02]

On 2020/01/24 17:01:50, hanwenn wrote:
> not LGtM - the setting is marked deprecated

actually, maybe this is the best solution for now - we otherwise have to write
autoconf detection for libgc to get the header that has the prototype for
GC_set_xxx.

[hanwenn, 2020-01-27 08:34:02]

rebase

[hanwenn, 2020-01-27 08:34:50]

dup issue.