Use EXEEXT_FOR_BUILD for makekeys

Use EXEXT_FOR_BUILD, to fix cross-compiling where EXEEXT differs from
EXEEXT_FOR_BUILD, such as when building for Windows from unix.

(Note: As written, this assumes EXEEXT_FOR_BUILD is always empty when
cross-compiling.  There could be some elaborate autodetection for
EXEXT_FOR_BUILD, but for the moment, if you are cross-compiling from
Windows to Unix, you'll need to set EXEEXT_FOR_BUILD explicity...)