Fix one of the most awesome "_MSC_VER means compiling with Microsoft's header set...

Fix one of the most awesome "_MSC_VER means compiling with Microsoft's header set" assumptions, revision 30728. Instead of breaking linking for user mode CRT to hide header incompatibilities we now use a less exciting solution; do not include the incompatible headers when implementing the affected functions -- abs.c and labs.c don't need anything at all, and strset.c only needs size_t, so define it there.

svn path=/trunk/; revision=43168