doxygen: add missing gb API doc generation

Files in include/osmocom/gprs/ and src/gb/ are not included in any doxygen
generated API docs. Add Doxyfile.gb.in and adjust configure.ac and Makefile.am.

Change-Id: Ieb64f497f55368e396872083237c9ff28da2dd93
diff --git a/Makefile.am b/Makefile.am
index 3f6df9b..b8de3ca 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,11 +19,14 @@
 
 html_DATA = $(top_builddir)/doc/html.tar
 
+doc: $(html_DATA)
+
 $(html_DATA): $(top_builddir)/doc/core/html/index.html \
 		  $(top_builddir)/doc/gsm/html/index.html \
 		  $(top_builddir)/doc/vty/html/index.html \
 		  $(top_builddir)/doc/codec/html/index.html \
-		  $(top_builddir)/doc/coding/html/index.html
+		  $(top_builddir)/doc/coding/html/index.html \
+		  $(top_builddir)/doc/gb/html/index.html
 	cd $(top_builddir)/doc && tar cf html.tar */html
 
 $(top_builddir)/doc/core/html/index.html: $(SOURCES) Doxyfile.core
@@ -50,13 +53,18 @@
 	mkdir -p doc/coding
 	$(DOXYGEN) Doxyfile.coding
 
+$(top_builddir)/doc/gb/html/index.html: $(SOURCES) Doxyfile.gb
+	@rm -rf doc/gb
+	mkdir -p doc/gb
+	$(DOXYGEN) Doxyfile.gb
+
 install-data-hook:
 	cd $(DESTDIR)$(htmldir) && tar xf html.tar && rm -f html.tar
 
 uninstall-hook:
-	cd $(DESTDIR)$(htmldir) && rm -rf {core,gsm,vty,codec,coding}
+	cd $(DESTDIR)$(htmldir) && rm -rf {core,gsm,vty,codec,coding,gb}
 
-DX_CLEAN = doc/{core,gsm,vty,codec,coding}/html/search/* doc/{core,gsm,vty,codec,coding}/{html,latex}/* doc/html.tar doc/{core,gsm,vty,codec,coding}/doxygen_sqlite3.db doc/*.tag
+DX_CLEAN = doc/{core,gsm,vty,codec,coding,gb}/html/search/* doc/{core,gsm,vty,codec,coding,gb}/{html,latex}/* doc/html.tar doc/{core,gsm,vty,codec,coding,gb}/doxygen_sqlite3.db doc/*.tag
 endif
 
 MOSTLYCLEANFILES = $(DX_CLEAN)