Fix for issue #43.

A file, UPC_VERSION, contains the UPC version number in the form
of 'X.X.X DATE', where X.X.X is like 3.4.1 and date like 20140327
(the date of the last update to llvm-upc/clang-upc/upc2c repositories).

By using "configure_file" on UPC_VERSION we force the cmake run on
every run of make, which will update the appropriate field in the
include/Basic/Version.inc file.