build: avoid requiring -lm

build: avoid requiring -lm

log2() is heavy when ffs() can do the same thing.  But ffs()
requires gnulib support for mingw.

This patch solves this linker error on Fedora 14.

/usr/bin/ld: libvirt_lxc-domain_conf.o: undefined reference to symbol 'log2@@GLIBC_2.2.5'
/usr/bin/ld: note: 'log2@@GLIBC_2.2.5' is defined in DSO /lib64/libm.so.6 so try adding it to the linker command line
/lib64/libm.so.6: could not read symbols: Invalid operation
collect2: ld returned 1 exit status

* .gnulib: Update to latest, for ffs.
* bootstrap.conf (gnulib_modules): Import ffs.
* src/conf/domain_conf.c (virDomainDefParseXML): Use ffs instead
of log2.
Reported by Dave Allan.