[Bug bootstrap/54304] linking stage picks up system mpfr instead of in-tree version

[tobi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54304

--- Comment #4 from Tobias Schlüter <tobi at gcc dot gnu.org> 2012-08-17 
19:29:56 UTC ---
BTW PR50914, though closed as invalid, indicates that similar mixups can also
happen on i686-linux.