Karsten Sperling authored Nov 17, 2016 and Brad King committed Nov 17, 2016

Callers of `_ep_get_build_command` do not initialize the output variable
they pass and expect the function to unconditionally set it.  Revise the
function to not check its own output variable.  Otherwise if a `cmd`
variable happens to be set when `ExternalProject_Add` is called then it
will be erroneously used as the default `BUILD`, `TEST`, and `INSTALL`
command.