Add a Configure probe for static inline.

Add a Configure probe for static inline.

This patch enables Configure to probe for C99-style 'static inline'.
(That is, functions may be inlined, but will not be externally visible.)
The initial idea is that some common code in messy macros inside headers
might be simplified using inline functions.  If the compiler does not
support 'static inline', then a plain 'static' is used instead, along
with the consequent implications of a function call (though the compiler
may optimize away the function call and inline the function anyway).
In either case, you simply use  PERL_STATIC_INLINE.

This patch does not *use* this facility at all yet.  It is merely a
Configure patch to make the facility availble for others to experiment
with.

VMS and Windows files will still need to be manually updated.

Finally, before actually converting anything to inline functions, please
try to carefully evaluate the performance implications of any proposed
changes.  Compilers vary in what they will and will not convert to inline
functions, so it's worth proceeding slowly and carefully.

This patch results from a single new metaconfig unit, d_static_inline.U,
which I will separately upload to the metaconfig repository.