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.
Merge pull request #7 from v4hn/wrong_foundation_check

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