When bumping the VERSION file, everything that includes the auto-generated
zeek/zeek-config.h needs to rebuild and ccache usage is voided due the file
changing. Split the mutable version information into a new zeek-version.h
file to avoid this.
Further, do not include zeek-version.h within Plugin.h and avoid the reference
to ZEEK_VERSION_FUNCTION unless we're building an external plugin.
Closes#2776.