adds proper dynamic linking

[lattice0]

dynamic linking was trying to link to a single file

this adds dynamic linking to the various .so files

tested and worked

[lattice0]

I also added multi architecture build. Sometimes when you want to build an Android APK you need a way of passing a single directory with .so files for every architecture.

Now with

FFMPEG_MULTI_ARCH_BUILD=1

FFMPEG_LIBS_DIR will be used as a base, and it will do like this

FFMPEG_LIBS_DIR/{target_os}/target_arch/

and it will search for the libs in this folder.

[lattice0]

would be nice to have this merged :)