Allow spl_config.h to be included by dependant packages

Allow spl_config.h to be included by dependant packages

We need dependent packages to be able to include spl_config.h so they
can leverage the configure checks the SPL has done.  This is important
because several of the spl headers need the results of these checks to
work properly.  Unfortunately, the autoheader build product is always
private to a particular build and defined certain common things.
(PACKAGE, VERSION, etc).  This prevents other packages which also use
autoheader from being include because the definitions conflict.  To
avoid this problem the SPL build system leverage AH_BOTTOM to include
a spl_unconfig.h at the botton of the autoheader build product.  This
custom include undefs all known shared symbols to prevent the confict.
This does however mean that those definition are also not availble
to the SPL package either.  The SPL package therefore uses the
equivilant SPL_META_* definitions.