m4: sasl: Use pkg-config
m4: sasl: Use pkg-config

Since commit 4e75b0a00fb4 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: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

eskultety/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,crobinso/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,eskultety/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,libvirt/libvirt,andreabolognani/libvirt,eskultety/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,jardasgit/libvirt,crobinso/libvirt,fabianfreyer/libvirt,zippy2/libvirt,eskultety/libvirt,andreabolognani/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,jfehlig/libvirt,jardasgit/libvirt,andreabolognani/libvirt,jfehlig/libvirt,andreabolognani/libvirt,eskultety/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt