m4: Improve portability for non-bash shells
m4: Improve portability for non-bash shells

= and == are both operators to test for string equality in bash,
but only = is required by POSIX.

Signed-off-by: Maya Rashish <e6362025b1bd435bf0a6fed19475c61f992cf03d@sdf.org>
Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

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