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.

.gnulib @ a918da4d

-Subproject commit 56005a21e8f9f434212a19dcb628c6d3b179fd08
+Subproject commit a918da4d61d28be61a12605c9d35e2cf3966d866

bootstrap.conf

@@ -36,6 +36,7 @@ count-one-bits
 crypto/md5
 dirname-lgpl
 fcntl-h
+ffs
 fnmatch
 func
 getaddrinfo

src/conf/domain_conf.c

@@ -29,7 +29,7 @@
 #include <fcntl.h>
 #include <dirent.h>
 #include <sys/time.h>
-#include <math.h>
+#include <strings.h>
 
 #include "virterror_internal.h"
 #include "datatypes.h"
@@ -5865,7 +5865,7 @@ static virDomainDefPtr virDomainDefParseXML(virCapsPtr caps,
             virDomainReportError(VIR_ERR_INTERNAL_ERROR,
                                  _("unexpected domain type %s, expecting %s"),
                                  virDomainVirtTypeToString(def->virtType),
-                                 virDomainVirtTypeToString(log2(expectedVirtTypes)));
+                                 virDomainVirtTypeToString(ffs(expectedVirtTypes) - 1));
         } else {
             virBuffer buffer = VIR_BUFFER_INITIALIZER;
             char *string;