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

 --- trunk/src/acos.c	2009/12/08 16:49:43	733

 +++ trunk/src/acos.c	2009/12/09 20:51:38	734

 @@ -188,10 +188,7 @@

    else

      rnd_im = rnd_im == GMP_RNDU ? GMP_RNDD

        : rnd_im == GMP_RNDD ? GMP_RNDU

 -#if MPFR_VERSION_MAJOR >= 3

 -      : rnd_im == GMP_RNDA ? GMP_RNDZ

 -#endif

 -      : rnd_im;

 +      : rnd_im; /* both RNDZ and RNDA map to themselves for -asin(z) */

    rnd1 = RNDC(GMP_RNDN, rnd_im);

    mpfr_init2 (pi_over_2, p);

    for (;;)