find /usr/include -name '*.h' | etags --declarations --no-globals
in your home directory and use the --include option of etags to include the generated TAG file in your home directory in your project TAG files.
This means you have a handy dictinonary you can use up to look up the function prototypes and macros in your system includes, which is something I have been missing a lot. Just looking at the function prototype itself is much better for jogging memory while in the zone, rather than having to hunt it down in a manual..