Avoid using AM_CONF_IF

Apparently this is a fairly recent feature of automake and is not
present on at least CentOS5.

libs/libzrtp/configure.in

@@ -89,8 +89,10 @@ if test -z "$DOXYGEN";
    then AC_MSG_WARN([Doxygen not found - continuing without Doxygen support])
 fi
 
-AM_CONDITIONAL([HAVE_DOXYGEN], [test -n "$DOXYGEN"])
-AM_COND_IF([HAVE_DOXYGEN], [AC_CONFIG_FILES([doc/Doxyfile])])
+if test -n "$DOXYGEN"; then
+  AM_CONDITIONAL([HAVE_DOXYGEN], [true])
+  AC_CONFIG_FILES([doc/Doxyfile])
+fi
 
 #
 # Generate Makefiles