Improvements to the libtool support in QEMU. Replace hard coded
libtool in the infrastructure with $(LIBTOOL) and allow
overriding the libtool binary used via the configure
script.

Reviewed-by: Andreas F=E4rber <andreas.faerber@web.de>
Signed-off-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>