In order to get an embedded build ID, add a new CMake cache variable
named OC_BUILD_ID. This variable can contain an arbitrary string, but
it is suggested that you use a hierarchical string starting with your
DNS domain, reversed, as in "com.example.openclonk.arbitrarystring"
(think java packages).
This assumes that pre-release versions will always have a x.y.90 version (or
higher), and will in those cases display the revision in place of the fourth
version segment.
Removed unused C4SHORTVERSION and C4EDITORCAPTION, replaced unset C4XVERBUILD
with C4XVER4, and more clearly separated the configuration from the derived
values.