The XINE check from here:
http://xine.cvs.sourceforge.net/viewvc/xine/xine-ui/configure.ac?revision=1.134&view=markup
looks like this:
dnl
dnl Checks for Xinerama extension
dnl
AC_ARG_ENABLE([xinerama],
   AS_HELP_STRING([--disable-xinerama], [Don't build support for 
xinerama (default: check)]))
if test "x$enable_xinerama" != "xno"; then
  PKG_CHECK_MODULES([XINERAMA], [xinerama], [ac_have_xinerama="yes"], [
    AC_CHECK_LIB(Xinerama, XineramaQueryExtension,
               XINERAMA_LIBS="-lXinerama"
               ac_have_xinerama="yes",,
               $X_LIBS $X_PRE_LIBS -lXext $X_EXTRA_LIBS)
  ])
  if test "x$ac_have_xinerama" = "xyes"; then
     X_PRE_LIBS="$X_PRE_LIBS $XINERAMA_LIBS"
     AC_DEFINE(HAVE_XINERAMA,,[Define this if you have libXinerama 
installed])
  fi
fi
AM_CONDITIONAL(HAVE_XINERAMA, test x$ac_have_xinerama = "xyes")
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