Added SDL2_net package.

[dkosmari]

This adds a package for SDL2_net.

It will, however, not work properly until devkitPro/wut#428 is merged into wut, since SDL2_net only uses select() to check for readability, and it triggers wut's bug in select(). The result is that SDLNet_SocketReady() will always return true, even when there's no data to read. Simply updating the wut's binary with the fix in that PR makes SDL2_net work properly, there's no need to rebuild SDL2_net.

The patch does modify configure.ac to remove bad usage of CFLAGS/LIBS, and was forcing libtool to pack all of libSDL2.a into libSDL2_net.a. This means that to build the package, autoconf, automake and libtool must be installed on the build system, since the build scripts will be generated again by running autoreconf.