configure.ac: add option "--disable-glib"

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

src/system/FatalError.cxx

@@ -23,7 +23,7 @@
 #include "util/Domain.hxx"
 #include "LogV.hxx"
 
-#ifdef WIN32
+#ifdef HAVE_GLIB
 #include <glib.h>
 #endif
 
@@ -78,12 +78,16 @@ FatalError(const char *msg, const Error &error)
 	FormatFatalError("%s: %s", msg, error.GetMessage());
 }
 
+#ifdef HAVE_GLIB
+
 void
 FatalError(const char *msg, GError *error)
 {
 	FormatFatalError("%s: %s", msg, error->message);
 }
 
+#endif
+
 void
 FatalSystemError(const char *msg)
 {

src/system/FatalError.hxx

@@ -45,9 +45,11 @@ gcc_noreturn
 void
 FatalError(const char *msg, const Error &error);
 
+#ifdef HAVE_GLIB
 gcc_noreturn
 void
 FatalError(const char *msg, GError *error);
+#endif
 
 /**
  * Call this after a system call has failed that is not supposed to