MacOSX is the second favourite development environment for Libav.
The tools required to build Libav are normally provided by Xcode with the exception of yasm.
nasm is known to not support targeting 64 bit MacOSX (macho64), please make sure yasm is installed.
Instructions regarding iOS support moved in a stand-alone page.
# brew install yasm
Is the suggested way to get a working assembler.
The normal ./configure && make would work just fine.
Third party ports systems make simpler to get libav installed.
Everything required is available out of box.
# brew install libav
Everything required is available out of box including libav from git using:
# emerge =libav-9999