title: ffmpeg null filter when configuring (no such filter 'eq')
body: <p>I configure ffmpeg to compile it an Android gcc (among others) like that:</p>
  
  <pre><code>./configure --disable-filters --enable-filter=atrim,trim,eq
  </code></pre>
  
  <p>And this is the preview:</p>
  
  <pre><code>Enabled filters:
  aformat          anull            atrim            format           null             trim
  </code></pre>
  
  <p>Obviously the <code>eq</code> filter is missing and in it's place it's the <code>null</code> enabled filter. As expected the <code>eq</code> filter is also missing from the executable binary and if we try use the <code>eq</code> filter it will return:</p>
  
  <pre><code> No such filter: 'eq', Error reinitializing filters!, Failed to inject frame into filter network: 
   Invalid argument, Error while processing the decoded data for stream #0:0, Conversion failed!
  </code></pre>
  
  <p>Full configuration:</p>
  
  <pre><code>--prefix=/path/to/x86 
  --enable-static --disable-shared 
  --enable-small 
  --enable-cross-compile 
  --target-os=linux
  --arch=x86 
  --sysroot=/path/to/sysroot 
  --cross-prefix=/path/to/i686-linux-android- 
  --enable-pic 
  --disable-ffplay 
  --disable-ffprobe 
  --disable-ffserver 
  --disable-doc 
  --disable-filters 
  --enable-filter=atrim,trim,eq 
  --disable-asm 
  --disable-debug
  --extra-cflags=-I/path/to/usr/include -O3 -Wno-deprecated-declarations -fpie -pie -Wl,-z,relro,-z,now -march=i686 -m32 -mtune=intel -mssse3 -mfpmath=sse
  --extra-ldflags=-L/path/to/usr/lib -fpie -pie
  </code></pre>