message: Add rc tag to BU version if rc is higher than 0
  I've tested it via using `make dist` and producing a gitian set of binaries. All the archives and the directories involved properly use the correct version, e.g. `BUcash-1.5.1.0rc2-linux64.tar.gz`
diff: @@ -13,11 +13,12 @@ define(_CLIENT_VERSION_MAJOR, 1)
  define(_CLIENT_VERSION_MINOR, 5)
  define(_CLIENT_VERSION_REVISION, 1)
  define(_CLIENT_VERSION_BUILD, 0) # version 99 here indicates an unreleased version
  +define(_CLIENT_VERSION_RC, 2)
  define(_CLIENT_VERSION_IS_RELEASE, true)
  define(_COPYRIGHT_YEAR, 2018)
  define(_COPYRIGHT_HOLDERS,[The %s developers])
  define(_COPYRIGHT_HOLDERS_SUBSTITUTION,[Bitcoin Bitcoin XT and Bitcoin Unlimited])
  -AC_INIT([Bitcoin Unlimited Cash Edition],[_CLIENT_VERSION_MAJOR._CLIENT_VERSION_MINOR._CLIENT_VERSION_REVISION._CLIENT_VERSION_BUILD],[https://github.com/BitcoinUnlimited/BitcoinUnlimited/issues],[BUcash])
  +AC_INIT([Bitcoin Unlimited Cash Edition],m4_join([.],_CLIENT_VERSION_MAJOR,_CLIENT_VERSION_MINOR,_CLIENT_VERSION_REVISION,_CLIENT_VERSION_BUILD)m4_if(_CLIENT_VERSION_RC, [0], [], [rc]_CLIENT_VERSION_RC),[https://github.com/BitcoinUnlimited/BitcoinUnlimited/issues],[BUcash])
  AC_CONFIG_SRCDIR([src/main.cpp])
  AC_CONFIG_HEADERS([src/config/bitcoin-config.h])
  AC_CONFIG_AUX_DIR([build-aux])
  