Makefile.am: distribute mpc.ico
configure.ac: improve detection of libgme

At least in my configuration libgme lacks .pc file
thus could not be detected with pkg-config.
mpdconf.example: improve documentation for replay gain

- Added replaygain_missing_preamp and replaygain_limit settings

- Added information about "auto" replay gain mode
mpdconf.example: provide an example of winmm output
mpdconf.example: provide an example of openal output
mpdconf.example: example improvements

- Add mandatory "name" setting for winmm/openal examples.

- Fix the language in openal sample (a -> an)
.gitignore: add src/win/mpd_win32_rc.rc

This file is generated during configuration and should be ignored.
decoder/ffmpeg: support float planar audio