Doxygen: enable tagfile generation and usage for cross-project references

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/Doxyfile.codec.in b/Doxyfile.codec.in
index 00ab43a..8435379 100644
--- a/Doxyfile.codec.in
+++ b/Doxyfile.codec.in
@@ -1485,12 +1485,12 @@
 # If a tag file is not located in the directory in which doxygen
 # is run, you must also specify the path to the tagfile here.
 
-TAGFILES               =
+TAGFILES               = doc/libosmocore.tag=../../core/html
 
 # When a file name is specified after GENERATE_TAGFILE, doxygen will create
 # a tag file that is based on the input files it reads.
 
-GENERATE_TAGFILE       =
+GENERATE_TAGFILE       = doc/libosmocodec.tag
 
 # If the ALLEXTERNALS tag is set to YES all external classes will be listed
 # in the class index. If set to NO only the inherited external classes