Turns out the current macros for gnu90-style inline
semantics stopped working with GCC 5
(and possibly also with 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.

See also
http://www.greenend.org.uk/rjk/tech/inline.html
for a write-up of gnu90 vs C99 semantics.

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 for MSVC anymore.

Reviewed By: erikd, austin

Differential Revision: https://phabricator.haskell.org/D2039