Fix GCC's "always_inline function might not be inlinable" warning.
As it turns out, these functions must also be marked as "inline"
if we want to enforce inlining.

Once at it, make the forced inline macro fall back to at least
APR-defined inlining (instead of no decoration at all) if there
is no way to enforce it.

* configure.ac
  (AH_BOTTOM): Make SVN__FORCE_INLINE imply APR_INLINE when not in
               SVN_DEBUG mode.

* subversion/svn_private_config.hw
  (SVN__FORCE_INLINE): Fall back to APR_INLINE in non-SVN_DEBUG mode
                       when not using Visual C.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1597962 13f79535-47bb-0310-9956-ffa450edef68