In order to quickly compile your  libx264, you can type the following commands in your shell:

$ cd /my/path/where/i/keep/compiled/stuff
$ git clone git://git.videolan.org/x264.git
$ cd x264
$ ./configure --enable-static --enable-shared
$ make
# make install
# ldconfig

Note: $ generally indicates that the command can be run as a normal user. # indicates that superuser privilages are generally required. If you are using Ubuntu, you'll most probably have to use sudo for commands preceded by #, such as:

$ sudo make install
$ sudo ldconfig

If you get a message like this:

Found yasm 0.x.x.xxxx
Minimum version is yasm-1.0.0
If you really want to compile without asm, configure with --disable-asm.

that means that you have an old version of yasm on your system, so you should update it. The quickest way to do so is like this:

$ wget http://www.tortall.net/projects/yasm/releases/yasm-1.2.0.tar.gz
$ tar xf yasm-1.2.0.tar.gz
$ cd yasm-1.2.0
$ ./configure
$ make
# make install

and after that, just repeat the configure, make, and make install steps for libx264 and it should be fine this time.