build: cppflags/ldflags conversion: libid3tag

This removes the rule for mp3_afh.o so that this object will instead
be created by means of the generic rule. To make this work, CPPFLAGS is
modified for this target to include suitable cpp options for libid3tag
as determined during configure.

Similarly, the linker flags for libid3tag are removed from
server_ldflags, afh_ldflags, play_ldflags, recv_ldflags and
play_ldflags, and appropriate linker flags are added to LDFLAGS for
all executables that need it.