Make logic for old-sprit-demanding compilers work.

The previous code was all broken, trying to mix conditional
requirements and indirect conditional requirements like:

    <toolset>borland:<conditional>@include-spirit


[SVN r43987]

libs/serialization

@@ -1 +1 @@
-Subproject commit ecd03e84d7a5d470c97802f1fcb561c69b58cca1
+Subproject commit cf8bc022f6928d39a981a366a8e546e190f26296