configure: fix --sysconfdir specification

--sysconfdir requires a parameter (the path), this should be reflected
in the case pattern.

Reported-by: Frank Arnold <frank.arnold@amd.com>
Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/configure b/configure
index abd0bf4..d728799 100755
--- a/configure
+++ b/configure
@@ -495,7 +495,7 @@
     static="yes"
     LDFLAGS="-static $LDFLAGS"
   ;;
-  --sysconfdir) sysconfdir="$optarg"
+  --sysconfdir=*) sysconfdir="$optarg"
   ;;
   --disable-sdl) sdl="no"
   ;;