configure.ac: make GLib optional by default

Only very few (minor) features still depend on GLib.

INSTALL

@@ -18,9 +18,6 @@ Any other C++11 compliant compiler should also work.
 
 Boost 1.46 - http://www.boost.org/
 
-GLib 2.28 - http://www.gtk.org/
-General-purpose utility library.
-
 
 Optional Output Dependencies
 ----------------------------

NEWS

@@ -10,6 +10,7 @@ ver 0.20 (not yet released)
 * mixer
   - null: new plugin
 * reset song priority on playback
+* remove dependency on GLib
 
 ver 0.19.5 (2014/11/26)
 * input

configure.ac

@@ -518,13 +518,18 @@ AM_CONDITIONAL(HAVE_ICU, test x$enable_icu = xyes)
 
 AC_ARG_ENABLE(glib,
 	AS_HELP_STRING([--enable-glib],
-		[enable GLib usage (default: enabled)]),,
+		[enable GLib (default: auto)]),,
-	enable_glib=yes)
+	enable_glib=auto)
+
+if test x$enable_glib != xno; then
+	PKG_CHECK_MODULES([GLIB], [glib-2.0 >= 2.28 gthread-2.0],
+		[found_glib=yes],
+		[found_glib=no])
+
+	MPD_AUTO_RESULT([glib], [Glib], [GLib not found])
+fi
 
 if test x$enable_glib = xyes; then
-	PKG_CHECK_MODULES([GLIB], [glib-2.0 >= 2.28 gthread-2.0],,
-		[AC_MSG_ERROR([GLib 2.28 is required])])
-
 	if test x$GCC = xyes; then
 		# suppress warnings in the GLib headers
 		GLIB_CFLAGS=`echo $GLIB_CFLAGS |sed -e 's,-I/,-isystem /,g'`
@@ -532,6 +537,7 @@ if test x$enable_glib = xyes; then
 
 	AC_DEFINE(HAVE_GLIB, 1, [Define if GLib is used])
 fi
+
 AM_CONDITIONAL(HAVE_GLIB, test x$enable_glib = xyes)
 
 dnl ---------------------------------------------------------------------------