ffmpeg -hide_banner -i Helicopter.mp4 -threads 0 -vf "scale=426:240" -c:v libx264 -preset slow -profile:v main -level 4 -g 60 -keyint_min 60 -x264-params "keyint=60:min-keyint=60:no-scenecut" -b:v 300k -maxrate 300k -bufsize 600k -r:v 30 -an -y -f mp4 Helicopter_240p_300.mp4

Unrecognized option 'x264-params'.
Error splitting the argument list: Option not found

I did this:
avconv -codecs | grep h264
avconv version 9.20-6:9.20-0ubuntu0.14.04.1, Copyright (c) 2000-2014 the Libav developers built on Dec  7 2016 21:27:42 with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04.3)
DEV.LS h264   H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (decoders: h264 h264_vdpau ) (encoders: libx264 )

The following are the enabled library of ffmpeg, but in my ffmpeg_build directory I have also x264 executable (it is also in the ujsr/local/bin directory). It seems I have not setted it in the properly way...

ffmpeg version N-86654-gd2ef9e6 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04.3)
  configuration: --disable-x86asm
  libavutil      55. 67.100 / 55. 67.100
  libavcodec     57.100.102 / 57.100.102
  libavformat    57. 75.100 / 57. 75.100
  libavdevice    57.  7.100 / 57.  7.100
  libavfilter     6. 94.100 /  6. 94.100
  libswscale      4.  7.101 /  4.  7.101
  libswresample   2.  8.100 /  2.  8.100

 I have followed the step by step guide of ffmpeg.org : https://trac.ffmpeg.org/wiki/CompilationGuide/Ubuntu

Could the problem be the configuration --disable-x86asm ?