m4: sasl: Use pkg-config

Since commit 4e75b0a0 we support SASL 2.1.26 and newer
releases only, all of which ship a .pc file. Using pkg-config
allows FreeBSD builds to pick up the dependency automatically.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

m4/virt-sasl.m4

@@ -22,7 +22,7 @@ AC_DEFUN([LIBVIRT_ARG_SASL],[
 ])
 
 AC_DEFUN([LIBVIRT_CHECK_SASL],[
-  LIBVIRT_CHECK_LIB([SASL], [sasl2], [sasl_client_init], [sasl/sasl.h])
+  LIBVIRT_CHECK_PKG([SASL], [libsasl2], [2.1.26])
 ])
 
 AC_DEFUN([LIBVIRT_RESULT_SASL],[