New cmake option LLVM_APPEND_VC_REV for controlling when the VC

revision id is appended to the LLVM version string. Defaults to
OFF.

Until now the VC revision id was always appended to the revision
string whenever cmake was invoked (either explicitly or implicitly
because a cmake source file changed). This was causing massive
recompilations because config.h are reconfigured with the new contents
of PACKAGE_VERSION.

llvm-svn: 122240