asterisk-16.x: allow eventfd detection

Asterisk 16 wants to use eventfd, but it doesn't allow the detection
during cross-compiling. This results in runtime warnings, for instance
when shutting down:

  [Mar  2 22:37:41] WARNING[21593]: alertpipe.c:112 ast_alertpipe_read: read() failed: Bad file descriptor
  [Mar  2 22:37:41] WARNING[21593]: alertpipe.c:112 ast_alertpipe_read: read() failed: Bad file descriptor
  [Mar  2 22:37:41] WARNING[21593]: alertpipe.c:112 ast_alertpipe_read: read() failed: Bad file descriptor

Relax the configure script so that eventfd can also be detected when
cross-compiling.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>

net/asterisk-16.x/patches/130-eventfd.patch

@@ -0,0 +1,11 @@
+--- a/configure.ac
++++ b/configure.ac
+@@ -1189,7 +1189,7 @@ if test "${ac_cv_have_variable_fdset}x"
+ fi
+
+ AC_MSG_CHECKING([if we have usable eventfd support])
+-AC_RUN_IFELSE(
++AC_LINK_IFELSE(
+   [AC_LANG_PROGRAM([#include <sys/eventfd.h>],
+       [return eventfd(0, EFD_NONBLOCK | EFD_SEMAPHORE) == -1;])],
+   AC_MSG_RESULT(yes)