Wrap etry and pop_callback functions in macros.

This allows them to display the filename and line number in the warning
when popping from an empty stack.

Although it would be nice to have the bitstream methods display that
also (e.g. unmark(), rewind(), free()), that's a bit too much hacking
for something that should never be seen in normal operation.
Have setup script populate -Idir arguments more aggressively.

This may wind up adding -I/usr/local/include several times, one for each
library we're using, but that shouldn't hurt anything.
Minor cleanup to library presence formatting.
Tweak library presence formatting a little further.

Avoid displaying library URL column at all if all the libraries are
present.
Adjust AudioOutput's set_format behavior.

Previously, it would always close and reopen the stream with the new
format.  Now it only reinitializes if the stream hasn't been initialized
or is in the wrong format.  This means the players can just call
.set_format() outright without checking .compatible() beforehand.
Shift ReplayGain text to the text module.

This is for track2track and tracktag which had been building those
messages from scratch.