MSVC: Place gendef.pl temporary file in the target directory.

MSVC: Place gendef.pl temporary file in the target directory.

Until now, it used the current working directory.  This makes it safe
for simultaneous invocations of gendef.pl, with different target
directories, to run from a single current working directory, such as
$(top_srcdir).  The MSVC build system will soon rely on this.

Christian Ullrich, reviewed by Michael Paquier.