Was using the wrong variable.  Regression by commit 16f870aa

Was hard-coded to "auto", and the macro parameter was ignored (in the
help text).

Broken by commit cdbdcec7

Simplify the definition of many build options.

Don't assign bash variables; instead, use the m4 parameters directly.

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.