Introduce a shared 'libosmogapk' library

The previous GAPK implementation was represented as a single
executable. So, all audio transcoding operations were available
only by calling the 'gapk' binary. This approach didn't allow
external applications to benefit from using GAPK API directly.

Since there are some projects (such as GR-GSM and OsmocomBB),
which are potential users of GAPK code base, it would be better
to have all transcoding functions within a shared library.

So, this change separates the common code into a shared library,
named 'libosmogapk', and links the 'gapk' binary against one.
Currently there are no shared headers, pkg-config manifest and
the export map, but they will be done latter.

src/Makefile.am

@@ -1,20 +1,90 @@
-AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
-AM_CFLAGS=-Wall $(LIBOSMOCODEC_CFLAGS) $(LIBOSMOCORE_CFLAGS) \
-	  ${OPENCORE_AMRNB_CFLAGS} $(LIBALSA_CFLAGS)
-AM_LDFLAGS=$(LIBOSMOCODEC_LIBS) $(LIBOSMOCORE_LIBS) \
-	   ${OPENCORE_AMRNB_LIBS} ${LIBGSM_LIBS} $(LIBALSA_LIBS)
+# This is _NOT_ the library release version, it's an API version.
+# Please read Chapter 6 "Library interface versions" of the libtool
+# documentation before making any modification
+LIBVERSION = 0:0:0
 
-COM_SOURCES = procqueue.c pq_file.c pq_format.c pq_codec.c pq_rtp.c pq_alsa.c \
-		formats.c fmt_amr.c fmt_gsm.c fmt_hr_ref.c fmt_racal.c \
-		fmt_amr_opencore.c \
-		fmt_rtp_amr.c fmt_rtp_efr.c fmt_rtp_hr_etsi.c fmt_rtp_hr_ietf.c \
-		fmt_rawpcm.c fmt_ti.c benchmark.c \
-		codecs.c codec_pcm.c codec_hr.c codec_fr.c codec_efr.c codec_amr.c
+AM_CPPFLAGS = \
+	$(all_includes) \
+	-I$(top_builddir) \
+	-I$(top_srcdir)/include \
+	$(NULL)
 
-bin_PROGRAMS = gapk
+AM_CFLAGS = \
+	-Wall \
+	$(LIBOSMOCORE_CFLAGS) \
+	$(LIBOSMOCODEC_CFLAGS) \
+	${OPENCORE_AMRNB_CFLAGS} \
+	$(LIBALSA_CFLAGS) \
+	$(NULL)
 
-gapk_SOURCES =	main.c $(COM_SOURCES)
+lib_LTLIBRARIES = libosmogapk.la
+
+libosmogapk_la_LDFLAGS = \
+	$(LIBOSMOCORE_LIBS) \
+	$(LIBOSMOCODEC_LIBS) \
+	${OPENCORE_AMRNB_LIBS} \
+	${LIBGSM_LIBS} \
+	$(LIBALSA_LIBS) \
+	-version-info $(LIBVERSION) \
+	-no-undefined \
+	$(NULL)
 
 if ENABLE_GSMHR
-gapk_LDADD = $(top_builddir)/libgsmhr/libgsmhr.la
+libosmogapk_la_LIBADD = $(top_builddir)/libgsmhr/libgsmhr.la
 endif
+
+# Processing queue implementation
+libosmogapk_la_SOURCES = \
+	procqueue.c \
+	pq_format.c \
+	pq_codec.c \
+	pq_file.c \
+	pq_alsa.c \
+	pq_rtp.c \
+	$(NULL)
+
+# Formats implementation
+libosmogapk_la_SOURCES += \
+	formats.c \
+	fmt_ti.c \
+	fmt_amr.c \
+	fmt_gsm.c \
+	fmt_hr_ref.c \
+	fmt_racal.c \
+	fmt_rawpcm.c \
+	fmt_rtp_amr.c \
+	fmt_rtp_efr.c \
+	fmt_rtp_hr_etsi.c \
+	fmt_rtp_hr_ietf.c \
+	fmt_amr_opencore.c \
+	$(NULL)
+
+# Codecs implementation
+libosmogapk_la_SOURCES += \
+	codecs.c \
+	codec_pcm.c \
+	codec_hr.c \
+	codec_fr.c \
+	codec_efr.c \
+	codec_amr.c \
+	$(NULL)
+
+# Codec benchmarking
+libosmogapk_la_SOURCES += \
+	benchmark.c \
+	$(NULL)
+
+# libosmogapk representative application
+bin_PROGRAMS = gapk
+
+gapk_SOURCES = \
+	main.c \
+	$(NULL)
+
+gapk_LDFLAGS = \
+	$(LIBOSMOCORE_LIBS) \
+	$(NULL)
+
+gapk_LDADD = \
+	$(top_builddir)/src/libosmogapk.la \
+	$(NULL)