libc/glibc: configure libc headers with support for 128-bit long double

At the headers install pass, there's no compiler to test for the size
of long double. Help it: we know that the compiler does support 128-bit
long double, in the IBM extended precision.

Kindly pointed out, and explained, by Mike FRYSINGER:
http://sourceware.org/ml/crossgcc/2009-11/msg00040.html
http://sourceware.org/ml/crossgcc/2009-11/msg00057.html

 #!/bin/sh

 canonicalizedname=$(readlink -nm "${0}")

 dirname="$(dirname "${canonicalizedname}")"

 basename="$(basename "${canonicalizedname}")"

 ld_lib_path="$(dirname "${dirname}")/lib"

 case ":${LD_LIBRARY_PATH}:" in

     *":${ld_lib_path}:"*)   ;;

     *)  LD_LIBRARY_PATH="${ld_lib_path}${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}";;

 esac

 export LD_LIBRARY_PATH

 exec "${dirname}/.${basename}" "$@"