libphobos: Fix link build errors when compiling with unsupported options

libphobos: Fix link build errors when compiling with unsupported options

The first compilation test to get baseline warnings was getting more
messages due to a missing object.d file, compared to later configure
tests where libphobos is in the include paths.

Because there must always be an object module during compilation, let
the tests themselves be an empty object module instead.

libphobos/ChangeLog:

2019-04-24  Iain Buclaw  <ibuclaw@gdcproject.org>

	PR d/88431
	* configure: Regenerate.
	* m4/libtool.m4 (lt_simple_compile_test_code): Update to not have
	dependencies on libphobos.
	(lt_simple_link_test_code): Likewise.
	(GDCFLAGS): Don't override for D compiler tests.

From-SVN: r270531