Make documentation builds reproducible

Doxygen includes the full path to sources in its documentation by default,
which makes builds non-reproducible.

Instruct it to use the shortest possible path that makes file names unique
instead.

doc/c/Doxyfile.in.in

@@ -158,7 +158,7 @@ INLINE_INHERITED_MEMB  = NO
 # shortest path that makes the file name unique will be used
 # The default value is: YES.
 
-FULL_PATH_NAMES        = YES
+FULL_PATH_NAMES        = NO
 
 # The STRIP_FROM_PATH tag can be used to strip a user-defined part of the path.
 # Stripping is only done if one of the specified strings matches the left-hand