Let the pkg-config stuff be overridden by variables

--- a/configure
+++ b/configure
@@ -14119,7 +14119,7 @@
         { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 $as_echo "yes" >&6; }
 
-  RAPTOR_VERSION=`$PKG_CONFIG raptor2 --modversion 2>/dev/null`
+  RAPTOR_VERSION=2.0.15
   raptor_too_old=0
   as_arg_v1=$RAPTOR_VERSION
 as_arg_v2=$RAPTOR_MIN_VERSION