Libguestfs - Jun 2021 - GSoC: Adding liburing dependency to libnbd

I think I can reuse the code below (libev part in configure.ac) and modify
it for liburing, there is the libxml2 part that provides an option to
disable libxml2 parts which I can reuse to provide similar behavior which
is simpler the without headers and lib checks, I feel libev metho

PKG_CHECK_MODULES([LIBEV], [libev], [
    AC_SUBST([LIBEV_CFLAGS])
    AC_SUBST([LIBEV_LIBS])
],[
    dnl no pkg-config for libev, searching manually:
    AC_CHECK_HEADERS([ev.h], [
        AC_CHECK_LIB([ev], [ev_time], [
            AC_SUBST([LIBEV_LIBS], ["-lev"])
        ],
        [
            AC_MSG_WARN([libev not found, some examples will not be
compiled])
        ])
    ],[
        AC_MSG_WARN([ev.h not found, some examples will not be compiled])
    ])
])

AS_IF([test "x$LIBEV_LIBS" != "x"], [
    old_CFLAGS="$CFLAGS"
    CFLAGS="$CFLAGS $LIBEV_CFLAGS -Werror=strict-aliasing -O2"
    AC_MSG_CHECKING([if the compiler is new enough for good aliasing rules])
    AC_COMPILE_IFELSE([
        AC_LANG_PROGRAM([
            #include <ev.h>

            static void cb (struct ev_loop *l, ev_timer *t, int e) { }
            static ev_timer timer;
        ], [
            ev_timer_init (&timer, cb, 0, .1);
        ])
    ], [
        AC_MSG_RESULT([yes])
    ], [
        AC_MSG_RESULT([no])
        LIBEV_CFLAGS="$LIBEV_CFLAGS -Wno-strict-aliasing"
    ])
    CFLAGS="$old_CFLAGS"
])
AM_CONDITIONAL([HAVE_LIBEV], [test "x$LIBEV_LIBS" != "x"])
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I went with libxml2 like for now
GitLab commit:
https://gitlab.com/rathod-sahaab/libnbd/-/commit/e9b90859c0a286d7023efd40080b4d88c978d083

AC_ARG_WITH([liburing],
    [AS_HELP_STRING([--without-liburing],
                    [disable use of liburing i.e. io_uring for
asynchronus io @<:@default=check@:>@])],
    [],
    [with_liburing=check])
AS_IF([test "$with_liburing" != "no"],[
    PKG_CHECK_MODULES([LIBURING], [liburing], [
        AC_SUBST([LIBURING_CFLAGS])
        AC_SUBST([LIBURING_LIBS])
        AC_DEFINE([HAVE_LIBURING],[1],[liburing found at compile time.])
    ], [
        AC_MSG_WARN([liburing not found, io_uring support disabled.])
    ])
])
AM_CONDITIONAL([HAVE_LIBURING], [test "x$LIBURING_LIBS" !=
"x"])
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