Add cache variables to link tests.

Add cache variables to link tests.

* libltdl/m4/libtool.m4 (_LT_SYS_DYNAMIC_LINKER)
(_LT_LINKER_SHLIBS): Add cache variables to tests that
require the linker to work.  For shlibpath_overrides_runpath,
this also changes the semantics to let the result from the C
compiler take precedence.
* tests/no-executables.at (AC_NO_EXECUTABLES): New file.
* Makefile.am: Update.
* NEWS: Update.
Report of shlibpath_overrides_runpath issue by Mike Frysinger,
report of GCC issue by Andreas Schwab.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>