libdfp: Clean configure.ac from GLIBC support

This patch cleans the remaning GLIBC build support from configure.ac.
It also setups the minimum autoconf to 2.63 (RHEL6/CENTOS6).
libdfp: Fix __SIZEOF_INT128__ dependency

Some GCC version do support __uint128_t, however they do not define
__SIZEOF_INT128__.  This patch adds a configure support to check if
the compiler actually check for 128-bits supports and sets
HAVE_UINT128_T define.