If you want to build CP/M Plus, the second and third links in the "CP/M 3" section at www.cpm.z80.de contain the CP/M 3 source tree and build scripts, for cross-compiling under DRDOS (second link) or UNIX (third link). 

In the case of the UNIX one in particular, it should just be a matter of installing [ZXCC][1] and [Thames][2], unpacking the source tree and typing 'make'.


  [1]: http://www.seasip.info/Unix/Zxcc/
  [2]: http://www.seasip.info/Unix/Thames/index.html