Explicitly specify obj_dir

[cmb69]

Without that, phpize builds fail because there is rar.c and rar.cpp, and both are compiled to rar.obj in the same folder. This appears to be an upstream bug[1], but even if so, that won't be fixed for PHP 7 anymore, so we work around.

[1] https://github.com/php/php-src/blob/php-8.1.0/win32/build/confutils.js#L1617-L1625