Log Message:
devel/php-pthreads: fix build problem

Fix build problem.  Since "lang/php/phpversion.mk" dose not pull in
"mk/pkg-build-options.mk", PKG_BUILD_OPTIONS.${PHP_PKG_PREFIX} was
always undefined.

And PKG_BUILD_OPTIONS.* is defined based on PKGBASE, check
PKG_BUILD_OPTIONS.php.