Fix missing HAVE_SOCKETS definition
Fix missing HAVE_SOCKETS definition

Certain systems such as Ubuntu Linux distribution with 3rd party
PHP repositories build and package PHP sockets extension separately
where the php_config.h file doesn't have the HAVE_SOCKETS defined.
This constant is used in the ext/sockets/php_sockets.h file and
therefore compilation fails with error:
unknown type name ‘php_socket’; did you mean ‘php_socket_t’

For these cases, a simple additional definition can be added via the
autotools build system.

This patch also checks for the presence of the PHP socket extension header
file and provides informational error message in the
./configure --enable-sockets
step directly if PHP sockets extension isn't present.

LinkedDestiny/swoole-src,swoole/swoole-src,swoole/swoole-src,LinkedDestiny/swoole-src,swoole/swoole-src,LinkedDestiny/swoole-src,swoole/swoole-src,LinkedDestiny/swoole-src,swoole/swoole-src,swoole/swoole-src,LinkedDestiny/swoole-src,LinkedDestiny/swoole-src,swoole/swoole-src,LinkedDestiny/swoole-src