move timespecsub compat out of ev.c

tweak the configure - more consistent naming for libs (LDADD_LIB_*) - allow to change CC and CFLAGS as arguments - allow to specify the LDADD_LIB_* as arguments

fmt

add -Wno-pointer-sign to the mix

add configure check for libmd; needed on some OSes for amused-web On the BSDs the SHA1Init() and friends are part of libc, while on linux (and maybe other systems as well) we need an implementation of these function: libmd for example.

split out the various LDADD_* so we don't link everything to everything

switch from libevent to a smaller, thin wrapper around poll() libevent is a very cool library, I like its APIs and enjoy using it. However, for amused it is maybe a bit too much since it doesn't deal with too many file descriptors. Amused only needs to monitor one fd for the player process and the currently connected clients. Given that it runs per-user locally, having more than a dozen of fds open is very, very rare.

add missing HAVE_IMSG in config.h otherwise we always use the bundled imsg regardless of its presence on the system. What's even worst is that we use the system header with the bundled version. Spotted in a OpenBSD package bulk following the libutil bump and imsg API change by tb@, diff is his. Thanks!

configure: ensure HAVE_SIO_FLUSH has a value when BACKEND=alsa

configure: allow to select the audio backend

configure: add -Wextra to the mix

grammar

copyright

configure: mention CC and CFLAGS in the help message

delete .d files produced by the tests