Support compilers defaulting to PIE

Support compilers defaulting to PIE

To support building glibc with GCC 6 configured with --enable-default-pie,
which generates PIE by default, we need to build programs as PIE.  But
elf/tst-dlopen-aout must not be built as PIE since it tests dlopen on
ET_EXEC file and PIE is ET_DYN.

	[BZ #17841]
	* Makeconfig (no-pie-ldflag): New.
	(+link): Set to $(+link-pie) if default to PIE.
	(+link-tests): Set to $(+link-pie-tests) if default to PIE.
	* config.make.in (build-pie-default): New.
	* configure.ac (libc_cv_pie_default): New.  Set to yes if -fPIE
	is default.  AC_SUBST.
	* configure: Regenerated.
	* elf/Makefile (LDFLAGS-tst-dlopen-aout): New.