Changed default doc version string.

docs/Doxyfile

@@ -24,7 +24,7 @@ PROJECT_NAME           = physfs
 # if some version control system is used.
 
 # (CMake will set this properly at build time. --ryan.)
-PROJECT_NUMBER         = unknown_version
+PROJECT_NUMBER         = "unknown version (build with 'make docs' next time!)"
 
 # The OUTPUT_DIRECTORY tag is used to specify the (relative or absolute) 
 # base path where the generated documentation will be put.