Error libx264 not found

It happens because you are compiling libav with --enable-libx264 which would need the libx264 headers to complete. It fails at the configure phase with:

configure: error: libx264 not found

This can be fixed easily installing the needed headers package libx264-dev which should be done with sudo apt-get install yasm libvpx. libx264. but for some reason it doesn't for you:

sudo apt-get install yasm libvpx. libx264.
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Note, selecting 'libvpx-doc' for regex 'libvpx.'
Note, selecting 'libvpx1-dbg' for regex 'libvpx.'
Note, selecting 'libvpx-dev' for regex 'libvpx.'
Note, selecting 'libvpx1' for regex 'libvpx.'
Note, selecting 'libx264-133' for regex 'libx264.'
Note, selecting 'libx264-142' for regex 'libx264.'
Note, selecting 'libx264-dev' for regex 'libx264.'
libvpx-dev is already the newest version.
libvpx1 is already the newest version.
libvpx1 set to manually installed.
yasm is already the newest version.
libx264-142 is already the newest version.
libx264-142 set to manually installed.
libx264-dev is already the newest version.
The following NEW packages will be installed:
  libvpx-doc libvpx1-dbg
0 upgraded, 2 newly installed, 0 to remove and 24 not upgraded.
Need to get 1,613 kB of archives.
After this operation, 8,027 kB of additional disk space will be used.
Do you want to continue? [Y/n] n
Abort.

include --enable-static --disable-opencl

eg -

cd ~/ffmpeg_sources
wget http://download.videolan.org/pub/x264/snapshots/last_x264.tar.bz2
tar xjvf last_x264.tar.bz2
cd x264-snapshot*
PATH="$PATH:$HOME/bin" ./configure --prefix="$HOME/ffmpeg_build" --bindir="$HOME/bin" --enable-static **--disable-opencl**
PATH="$PATH:$HOME/bin" make
make install
make distclean