Description of problem:
If Events APIs are disabled using `./configure --disable-events` then make will get following error since events.h or events.c files are not loaded in libglusterfs. I think we need to use #if (USE_EVENTS) and #endif everywhere we call gf_event

Additional info:
cli-cmd-volume.c: In function ‘cli_cmd_volume_create_cbk’:
cli-cmd-volume.c:247:17: error: implicit declaration of function ‘gf_event’ [-Werror=implicit-function-declaration]
                 gf_event (EVENT_VOLUME_CREATE, "name=%s", (char *)words[2]);
                 ^
cli-cmd-volume.c:247:27: error: ‘EVENT_VOLUME_CREATE’ undeclared (first use in this function)
                 gf_event (EVENT_VOLUME_CREATE, "name=%s", (char *)words[2]);
                           ^
cli-cmd-volume.c:247:27: note: each undeclared identifier is reported only once for each function it appears in
cli-cmd-volume.c: In function ‘cli_cmd_volume_delete_cbk’:
cli-cmd-volume.c:324:27: error: ‘EVENT_VOLUME_DELETE’ undeclared (first use in this function)
                 gf_event (EVENT_VOLUME_DELETE, "name=%s", (char *)words[2]);
                           ^
cli-cmd-volume.c: In function ‘cli_cmd_volume_start_cbk’:
cli-cmd-volume.c:402:27: error: ‘EVENT_VOLUME_START’ undeclared (first use in this function)
                 gf_event (EVENT_VOLUME_START, "name=%s", (char *)words[2]);
                           ^
cli-cmd-volume.c: In function ‘cli_cmd_volume_stop_cbk’:
cli-cmd-volume.c:538:27: error: ‘EVENT_VOLUME_STOP’ undeclared (first use in this function)
                 gf_event (EVENT_VOLUME_STOP, "name=%s", (char *)words[2]);
                           ^
  CC       cli-cmd-peer.o
cc1: some warnings being treated as errors
Makefile:549: recipe for target 'cli-cmd-volume.o' failed
make[3]: *** [cli-cmd-volume.o] Error 1
make[3]: *** Waiting for unfinished jobs....
cli-cmd-peer.c: In function ‘cli_cmd_peer_probe_cbk’:
cli-cmd-peer.c:94:17: error: implicit declaration of function ‘gf_event’ [-Werror=implicit-function-declaration]
                 gf_event (EVENT_PEER_ATTACH, "host=%s", (char *)words[2]);
                 ^
cli-cmd-peer.c:94:27: error: ‘EVENT_PEER_ATTACH’ undeclared (first use in this function)
                 gf_event (EVENT_PEER_ATTACH, "host=%s", (char *)words[2]);
                           ^
cli-cmd-peer.c:94:27: note: each undeclared identifier is reported only once for each function it appears in
cli-cmd-peer.c: In function ‘cli_cmd_peer_deprobe_cbk’:
cli-cmd-peer.c:168:27: error: ‘EVENT_PEER_DETACH’ undeclared (first use in this function)
                 gf_event (EVENT_PEER_DETACH, "host=%s", (char *)words[2]);
                           ^
cc1: some warnings being treated as errors