Refine amr build

This pull-request starts huge series of (future) pull-requests by me to clean-up oph build system.

This pull-request introduces (hopefully) universal Makefile (now for amr codec only) that produces builtin.o blob instead of static .a library. Similar makefiles should be used for each codec further.

Also this pull-request wipes out test apps (encoder.c and decoder.c) that are not involved in oph build process.

Please test how this Makefile beaves under mingw.