I run into obvious barrier of ffmpeg API changes.

For easy install I just cloned mplayer2 code from github first

git clone https://github.com/astiob/mplayer2.git

Then downloaded ffmpeg 2.3.6 tar.bz2 unpacked and configured it on 32 bit system like this

# ./configure --disable-debug --disable-asm --disable-doc --enable-avresample

May be you can leave asm enabled on 64 bit x86-64 system, for me it failed in h264 header.

Then I installed this ffmpeg to default /usr/local prefix 

Then I configured mplayer2 without sdl:

./configure --disable-sdl

And then I run into problem ...for some reason any ffmpeg decoded video was failing to init vo .. even  null vo!

I finally guessed there was missing {} pair ...

After patching mplayer2 sources with attached patch I finally got output!

I also slightly enlarged help text array, so

-vo gl3:help will not truncated in the middle ...

Note, by default mplayer2 binary still named mplayer, so please do not install this mplayer over system provided one, just run it from its source dir