Missing POST_BUILD in add_custom_command

[eyalroz]

Each add_custom_command() (in CMake 3.1 and later at least) must specify its priority w.r.t. other rules for the target, through the use of PRE_BUILD, POST_BUILD etc. This is missing in the add_custom_command() in blitz/CMakeLists.txt, which gets us the following warning:

CMake Warning (dev) at blitz/CMakeLists.txt:76 (add_custom_command):
  Exactly one of PRE_BUILD, PRE_LINK, or POST_BUILD must be given.  Assuming
  POST_BUILD to preserve backward compatibility.

  Policy CMP0175 is not set: add_custom_command() rejects invalid arguments.
  Run "cmake --help-policy CMP0175" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

I suggest it be marked with POST_BUILD - assuming that was the original intent, perhaps with an earlier version of CMake.