format_mp3: Re-work menuselect/build issues

Rather than removing format_mp3 from ALL_C_MODS (which caused format_mp3
to not show up in menuselect), use .PHONY targets when the necessary
source files are not present.

ASTERISK-23951
Reported by: Tzafrir Cohen

Change-Id: I0a7512c51acc9e86043671795020b0de725bd9e8

addons/Makefile

@@ -32,12 +32,9 @@ ALL_C_MODS:=app_mysql \
             cdr_mysql \
             chan_mobile \
             chan_ooh323 \
+            format_mp3 \
             res_config_mysql
 
-ifneq ($(wildcard mp3/Makefile),)
-	ALL_C_MODS += format_mp3
-endif
-
 all: check_mp3 _all
 
 check_mp3:
@@ -65,6 +62,10 @@ include $(ASTTOPDIR)/Makefile.moddir_rules
 chan_ooh323.so: _ASTCFLAGS+=$(H323CFLAGS)
 $(call MOD_ADD_C,chan_ooh323,$(H323SOURCE))
 
+ifneq ($(wildcard mp3/Makefile),)
 $(call MOD_ADD_C,format_mp3,mp3/common.c mp3/dct64_i386.c mp3/decode_ntom.c mp3/layer3.c mp3/tabinit.c mp3/interface.c)
 
 .PHONY: check_mp3
+else
+.PHONY: check_mp3 format_mp3.o format_mp3.so
+endif