Use of ldexp() requires -lm on some platforms; use gnulib to determine
this for our makefile.  Also, optimize virRandomInt() for the case
of a power-of-two limit (actually rather common, given that Daniel
has a pending patch to replace virRandomBits(10) with code that will
default to virRandomInt(1024) on default SELinux settings).

* .gnulib: Update to latest, for ldexp.
* bootstrap.conf (gnulib_modules): Import ldexp.
* src/Makefile.am (libvirt_util_la_CFLAGS): Link with -lm when
needed.
* src/util/virrandom.c (virRandomInt): Optimize powers of 2.