php: add PHP_VERSION_MAJOR variable

Add PHP_VERSION_MAJOR variable to php.mk for use by other packages.
E.g. to install the Ioncube loader binary, it is necessary to
know the PHP major version.

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>