Fix build/17104

Fix build/17104

This patch is to fix the build error when GDB is configured as:

  CFLAGS=-Wall ./configure --with-babeltrace; make

This patch adds one line of code in configure test to use local
variable 'pos'.

Note that we append -Werror to CFLAGS to catch the warning related to
assignment to scope.  See more in this thread
https://sourceware.org/ml/gdb-patches/2014-08/msg00045.html

2014-08-13  Yao Qi  <yao@codesourcery.com>

	PR build/17104
	* configure.ac: Use local variable 'pos'.
	* configure: Regenerated.