libstdc++: Move test for __cpp_lib_not_fn to version.cc

libstdc++: Move test for __cpp_lib_not_fn to version.cc

When running the tests without pre-compiled headers
(--disable-libstdcxx-pch), the test fails, because the feature
testing macro (FTM) isn't defined yet.

This commit moves checking the FTM to a dedicated file (version.cc)
that's run without PCH.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/function_objects/not_fn/nttp.cc: Move
	test of feature testing macro to version.cc
	* testsuite/20_util/function_objects/not_fn/version.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>