configure.ac: respect 'g' variants of libtool/libtoolize

BSD OS'es (OS X for me) provide GNU tools with prefixed 'g'. To find correct
versions of libtool/libtoolize on those systems search also for
glibtool/glibtoolize.

Signed-off-by: Andreas Bießmann <andreas@biessmann.de>
Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
[yann.morin.1998@free.fr: search for the g-variant first]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <2f1530b54afcb6a00e1d.1395158786@andreas-mbp.er.biessmann.org>
Patchwork-Id: 331467

 # Dummy config file to not use a C library *at all*

 ## depends on BARE_METAL

 ##

 ## help Do not use a C library.

 ## help

 ## help This is usefull if your project is self-contained, does not rely on

 ## help an external C library, and provides all the necessary bits.

 ## help

 ## help Most probably usefull to bootloaders, as they generally don't depend

 ## help on the C library.

 ## help

 ## help If unsure: do *not* choose that, and use another option in the choice.