regex: avoid glibc deadlock during configure

regex: avoid glibc deadlock during configure

glibc has a known bug where certain corruptions of the heap can
cause malloc to default to printing a debug message that includes
a backtrace, but the act of getting the backtrace uses dlopen
which in turn calls into malloc, causing a recursive lock ending
in deadlock.  Thus, when configure is probing for a known glibc
heap corruption bug, the overall configure would hang.

The solution suggested by glibc developers is to force malloc
to quit printing debug messages, which avoids recursive malloc.

* m4/regex.m4 (gl_REGEX): Avoid recursive malloc deadlock when
glibc bug 15078 in turn triggers bug 16159.
Reported by Michal Privoznik.

Signed-off-by: Eric Blake <eblake@redhat.com>