Rationale: vanilla libid3tag does not have any pkg-config stuff
and fails to detect because symbols from libz are not found.

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

This function checks whether a prerequisite for a feature was found.

When MPD_AUTO_DISABLED prints a fatal error message, include the
feature name.  This might be an important piece of information for the
user, just in case the preceding line doesn't tell him.

In the MPD_AUTO_DISABLED() function, I forgot to check for "explicitly
disabled".

MPD_AUTO_PKG() runs pkg-config and then forwards control to
MPD_AUTO_RESULT().  This is a commonly used short cut.

This patch adds a small autoconf M4 library which deals with
auto-detected features.  The default for those features is "auto",
which is like the old default: if the library is present on the
system, enable the feature, disable otherwise.  If the user explicitly
enables that feature (--enable-alsa), and the library is not present,
configure must fail, because it cannot fulfill the request.