Select I/O queue backend using macro PJ_IOQUEUE_IMP

[nanangizz]

Currently switching an I/O queue backend need to re-run configure for GNU build system or modify the PJLIB project for Visual Studio. With this PR, the backend can be selected by setting macro PJ_IOQUEUE_IMP in config_site.h, the default is PJ_IOQUEUE_IMP_SELECT or using select(). Available backends are:

pjproject/pjlib/include/pj/config.h

Lines 699 to 718 in b3c8a81

	/** No/dummy I/O queue */
	#define PJ_IOQUEUE_IMP_NONE 0
	/** Using select() */
	#define PJ_IOQUEUE_IMP_SELECT 1
	/** Using epoll() (experimental) */
	#define PJ_IOQUEUE_IMP_EPOLL 2
	/** Using Windows I/O Completion Ports (experimental) */
	#define PJ_IOQUEUE_IMP_IOCP 3
	/** Using MacOS/BSD kqueue (experimental) */
	#define PJ_IOQUEUE_IMP_KQUEUE 4
	/** Using Windows UWP socket (deprecated) */
	#define PJ_IOQUEUE_IMP_UWP 5
	/** Using Symbian (deprecated) */
	#define PJ_IOQUEUE_IMP_SYMBIAN 6

The existing configure params, --enable-epoll & --enable-kqueue, will still work.