Handle tests-unsupported if run-built-tests = no.

The tests-unsupported variable lists tests that should neither be
compiled nor run, because some support needed to compile them is
missing.

The implementation of this feature involves having a rule to create
.out files for these tests that takes precedence over the default
rule.  This does not work in the run-built-tests = no case (cross
compiling without use of a wrapper to run the tests on a separate
system, in which cases most tests are compiled only) because in that
case the tests target depends on $(tests) to ensure all tests get
compiled.  This patch changes that dependency to filter out
$(tests-unsupported).

Tested with cross-compilation to ARM with GCC 5, where libstdc++ is
missing some C++11 support because of the bug I fixed in
<https://gcc.gnu.org/ml/gcc-patches/2015-10/msg01040.html> and so
tests-unsupported is nonempty and the tests in question fail to
compile.  (When I originally observed the bug, it was with a native
build / test simply using an x86_64 compiler that had been configured
as a cross compiler to isolate it from the system headers / libraries,
so the configuration issue applied to the compiler but run-built-tests
was yes, so I don't observe the issue with tests-unsupported with that
compiler.)

	* Rules [$(run-built-tests) = no] (tests): Do not depend on
	$(tests-unsupported).