Merge config/ changes from GCC.
config:
Merge from GCC:
2018-10-28 Iain Buclaw <ibuclaw@gdcproject.org>
* multi.m4: Set GDC.
2018-07-05 James Clarke <jrtc27@jrtc27.com>
* dfp.m4 (enable_decimal_float): Enable for x86_64*-*-gnu* to
catch x86_64 kFreeBSD and Hurd.
libdecnumber:
* configure: Regenerate.
zlib:
* configure: Regenerate.