Add a new optional dependency on the miniupnpc library.

The miniupnpc library is a lightweight UPnP-IGD client.

http://miniupnp.free.fr/

Contrary to other libraries, this dependency is disabled by default.
This is because the library is somewhat obscure and is only tangentially
useful, so enabling it by default would probably annoy most users.

m4/miniupnpc.m4

@@ -0,0 +1,40 @@
+dnl Check to find the miniupnpc headers/libraries
+
+AC_DEFUN([tinc_MINIUPNPC],
+[
+  AC_ARG_ENABLE([miniupnpc],
+    AS_HELP_STRING([--enable-miniupnpc], [enable miniupnpc support]))
+  AS_IF([test "x$enable_miniupnpc" = "xyes"], [
+  AC_DEFINE(HAVE_MINIUPNPC, 1, [have miniupnpc support])
+    AC_ARG_WITH(miniupnpc,
+      AS_HELP_STRING([--with-miniupnpc=DIR], [miniupnpc base directory, or:]),
+      [miniupnpc="$withval"
+       CPPFLAGS="$CPPFLAGS -I$withval/include"
+       LDFLAGS="$LDFLAGS -L$withval/lib"]
+    )
+
+    AC_ARG_WITH(miniupnpc-include,
+      AS_HELP_STRING([--with-miniupnpc-include=DIR], [miniupnpc headers directory]),
+      [miniupnpc_include="$withval"
+       CPPFLAGS="$CPPFLAGS -I$withval"]
+    )
+
+    AC_ARG_WITH(miniupnpc-lib,
+      AS_HELP_STRING([--with-miniupnpc-lib=DIR], [miniupnpc library directory]),
+      [miniupnpc_lib="$withval"
+       LDFLAGS="$LDFLAGS -L$withval"]
+    )
+
+    AC_CHECK_HEADERS(miniupnpc/miniupnpc.h,
+      [],
+      [AC_MSG_ERROR("miniupnpc header files not found."); break]
+    )
+
+    AC_CHECK_LIB(miniupnpc, upnpDiscover,
+      [MINIUPNPC_LIBS="$LIBS -lminiupnpc"],
+      [AC_MSG_ERROR("miniupnpc libraries not found.")]
+    )
+  ])
+
+  AC_SUBST(MINIUPNPC_LIBS)
+])