Libav builds on a large number of platforms and its build system allows a precise tuning of the components to enable or disable, allowing special purpose builds for constrained environments or legally-restricted nations.
Libav supports a good number of external decoders.
Libav uses pkg-config/pkgconf when available to check for most of the external components.
Make sure the libraries .pc files are reachable by extending your PKG_CONFIG_PATH.
# Libraries in a non-standard path PKG_CONFIG_PATH=/the/path/lib/pkgconfig ./configure --enable-libx264
--disable-all will turn off all the libraries, you must pass explicit --enable-avutil in order to have --enable-avcodec and so on.