Revert "disable building shared libs for graphviz, add clarifying comments"

This reverts commit a0adb536b80052f72b23edd26b3271fc6e1f8e01.
Revert "disable building of shared Pango libs, since it requires cairo (and thus GCC) to be built with -fPIC"

This reverts commit e8ddd580597402961c599a751b4f0abd08b8d007.
Revert "disable building of shared cairo libs, since it requires GCC to be built with -fPIC"

This reverts commit 348630d2fcdce65b0b7c825ec8b4718399ae2292.