mpfr / gmp dependency

[NightStrike]

I've been playing around with gcc 6 rc2, and I found that when I build
with a very high make -j on a system with a large number of cores
(64), there's a dependency breakage between gmp and mpfr.  The mpfr
configure only correctly detects certain features of gmp after gmp has
been fully built (all-gmp completed).  Otherwise, I get things like
undefined symbols for mpfr_clz_tab, which are based on mpfr
configure's detection of what gmp has available.

When I don't try to parallelize it and just build gmp fully before
building mpfr (though I do a "make all-gmp -j; make all-mpfr -j"),
then everything works fine.

Is this something that I should put in bugzilla, or am I doing it
wrong, and I shouldn't expect parallel make with very high values of j
to work?