Add lib/gvc to CMake build

Add lib/gvc to CMake build

The dynamic library `gvc` is the core library of Graphviz. It has various
dependencies, of which some are third party libraries. The DLLs of these
libraries will be included in the package on Windows.

Now also generating `builddate.h` on all platforms. The Windows Visual
Studio build didn't generate this file.

Now also defines GVPLUGIN_CONFIG_FILE, set to confg6, just like the
Autotools build.

Support the "enable-ltdl" option, default value is ON.

Now passing the "-fPIC" to all compilers, except MSVC.

The following checks have been added:
- FindLTDL
- FindRxSpencer (on Windows only)