Poco detection is too confident when seeing /usr/include/Foundation

That folder is also provided by gnustep-base.
Ubuntu encapsulates the gnustep headers in a GNUstep folder,
however the project itself installs to /usr/include/Foundation
and other linux distributions(Archlinux, Gentoo, Lunar-Linux, ...)
respect this.
Check for Poco/SharedLibrary.h instead of arbitrary AbstractCache.h

SharedLibrary.h is included in the code, so it makes sense
to check for this header specifically.
fix inconsistency in PocoConfig.cmake leading to CMake warning (regression of ab035a58a6306c67dd16c75b1917e0ab882e4a02)