CXXFeatures: include <initializer_list> in it's testcase
CXXFeatures: avoid checking nullptr_t in nullptr test

std::nullptr_t is in <cstddef>, which means the test will fail with a
supporting compiler but old headers. nullptr itself should work without any
headers.
CXXFeatures: allow test results to differ dependin on headers

Some features depend on support from the C++ library, not only the compiler.
Merge topic 'cxx11' into next

64405bb CXXFeatures: allow test results to differ dependin on headers
262716b CXXFeatures: avoid checking nullptr_t in nullptr test
74ae500 CXXFeatures: include <initializer_list> in it's testcase