CMake: Generate & install graphviz_version.h

CMake: Generate & install graphviz_version.h

Don't encode build date stamp in it to be able to create reproducable
builds. Slightly different to the autotools version:

Here's the diff to to the autotools based version:
```
---
/home/kfunk/opt/graphviz-autotools/include/graphviz/graphviz_version.h
2017-06-13 22:16:17.447416244 +0200
+++ graphviz_version.h  2017-06-13 22:27:29.343298754 +0200
@@ -2,7 +2,7 @@
 #define GVPLUGIN_VERSION 6
 #define PACKAGE_BUGREPORT "http://www.graphviz.org/"
 #define PACKAGE_NAME "graphviz"
-#define PACKAGE_STRING "graphviz 2.41.20170613.1957"
+#define PACKAGE_STRING "graphviz 2.41.0"
 #define PACKAGE_TARNAME "graphviz"
 #define PACKAGE_URL ""
-#define PACKAGE_VERSION "2.41.20170613.1957"
+#define PACKAGE_VERSION "2.41.0"
```