Remove references to "skip-native" as it was removed a long time ago

... replaced it with the "-native" property

build.xml

@@ -12,7 +12,7 @@
        (cross-compile currently only configured/tested on w32ce-arm and
        android targets) 
 
-       Use ANT_OPTS=-Dskip-native=false to build native parts, or directly
+       Use ANT_OPTS=-D-native=true to build native parts, or directly
          invoke the native or test targets
        Use ANT_OPTS=-Dheadless to run tests headless
        Use ANT_OPTS=-Drelease to stage a final, non-snapshot version

www/Contributing.md

@@ -17,7 +17,8 @@ gcc, autotools (for libffi), ant (1.8+), a JDK (1.4+), and a few other typical
 command-line utilities available.  Feel free to report any issues, we'll
 generally pull build fixes immediately. 
 
-Native bits are built by passing `-Dskip-native=false` to `ant`.  It's
+Native bits are built by invoking `ant native`.  The build system is configured
+to rebuild the native library automaticly if necessary. It's
 safe to skip the native build as long as your modifications are restricted to
 Java code.