Configure: add check on headers

Based on autoscan report:

configure.ac: warning: missing AC_CHECK_HEADERS([fcntl.h]) wanted by: lib/context.c:44
configure.ac: warning: missing AC_CHECK_HEADERS([limits.h]) wanted by: include/babeltrace/compat/limits.h:26
configure.ac: warning: missing AC_CHECK_HEADERS([netdb.h]) wanted by: formats/lttng-live/lttng-live-comm.c:27
configure.ac: warning: missing AC_CHECK_HEADERS([netinet/in.h]) wanted by: formats/lttng-live/lttng-live-comm.c:26
configure.ac: warning: missing AC_CHECK_HEADERS([stddef.h]) wanted by: include/babeltrace/compiler.h:28
configure.ac: warning: missing AC_CHECK_HEADERS([sys/socket.h]) wanted by: formats/lttng-live/lttng-live-comm.c:24

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>