Turns out the current macros for gnu90-style inline
semantics stopped working with GCC 5
(and possibly also Apple's GCC) which switched on
GNUC_STDC_INLINE by default falling back to using the
suboptimal static inline mode.
However, C99 supports an equivalent (as far as our
use-case is concerned) extern inline mode.
This patch also removes the MSVC case as VS2015 is supposed
to finally catch up to C99 (and C11), so we don't need any
special care anymore.