To replace libcue, the unmaintained and crashy library.

Replace -I with -isystem in GLIB_CFLAGS.

It appears the musepack section has not really been reviewed in some
time, many parts unfunctional, others, just unnecessary.

Ensure that WINVER is defined early enough, so other system headers
won't fall back to their default value.  Specifically, this solves a
build failure (-Werror) with mingw-w64 ("WINVER redefined").

Prepare for enabling "silent-rules".

Don't use MPD_LIBS, MPC_CFLAGS.

Don't use MPD_LIBS.

Use DESPOTIFY_LIBS in Makefile.am instead.  Also, use
DESPOTIFY_CFLAGS.

gprof is obsolete, it's better to use perfcounters in every aspect.

AC_SEARCH_LIBS allows the function to be present in libc.

One call to AC_SEARCH_LIBS replaces AC_CHECK_FUNCS and AC_CHECK_LIB.

Defaulting to "with systemd" causes problems for users who install MPD
as an unprivileged user, and it breaks "make distcheck".  It looks
like enabling it by default creates too many practical problems for
unexperienced users.

With --with-systemdsystemunitdir (without a parameter), configure.ac
attempts to auto-detect systemd.

Allow enabling the plugin explicitly without running Solaris, to test
the build.

Wrong variable name.

Add -DNDEBUG to AM_CPPFLAGS.

libOggFLAC has been deprecated for 5 years now, and we havn't been
testing it for a long time.  Let's delete it.

.. and a minor tweak for libFLAC+libogg detection.

The pkg-config file was added by the Debian package maintainers, and
unfortunately, the rest of the world doesn't have it.

Add M4 function MPD_AUTO_PKG_LIB for pkg-config with AC_CHECK_LIB
fallback.

Don't abort the configure script when avahi could not be
auto-detected.  It previously did, because there was no custom "fail"
action for PKG_CHECK_MODULES.