Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.

We want to remove gcc.h eventually. This takes care of all the
G_GNUC_MALLOC macros.

Due to a minor typo, the string set had duplicate values, because
strset_add() didn't check the base slot properly.

"struct strset" is a hashed string set: you can add strings to this
library, and it stores them as a set of unique strings.  You can get
the size of the set, and you can enumerate through all values.

This will be used to replace the linear tagTracker library.