build: Generate module symdefs in src/modules directory
This will make it possible to remove the empty Makefile.am files.
- module-...-symdef.h files are all generated in src/modules, instead of in the subdir of the module.
- The default inclusion of src/modules subdirs in AM_CFLAGS can be removed, where necessary (raop) the subdir is
included in the specific CFLAGS.
- The src/daemon and src/modules directories are always created on make, to facilitate out of tree builds.
- AM silent rules are used for the generation of symdef files by m4.
- For echo-cancel, keep the build dir include for now (and mkdir it) although limit it to just the echo-cancel
module's CFLAGS (Colin Guthrie)