Define missing macros span_HAVE_INLINE_NAMESPACE and span_CONFIG_CONTRACT_VIOLATION_THROWS_V

[feroldi]

GCC (under -Werror=undef) whines about span_HAVE_INLINE_NAMESPACE and span_CONFIG_CONTRACT_VIOLATION_THROWS_V macros not being defined. The former one was actually missing a proper definition (C++11 and MSVC 14.0 have inline namespaces). The latter was a misuse: checking for an undefined macro's value.

I made the changes myself, but I'm not sure whether I should PR this.

[martinmoene]

Thanks @feroldi , I took care of it in the two commits mentioned above.

[feroldi]

Thanks!