Only link to a subset of the Poco libraries to avoid name clashes.

PocoPDF contains a libpng copy whose symbols may lead to name clashes
when linking a target to QtGui (for example) which depends on libpng
and to PocoPDF. In some cases, the linker will resolve the png symbol
references in QtGui to point to the ones in PocoPDF, leading to runtime
crashes due to incompatible symbol versions.