make(java): Migrate samples to maven (#202)

- Factorise makefile code using advanced make function
- Use the Loader in samples

ortools/sat/samples/OptionalIntervalSampleSat.java

@@ -13,6 +13,7 @@
 
 package com.google.ortools.sat.samples;
 
+import com.google.ortools.Loader;
 import com.google.ortools.sat.CpModel;
 import com.google.ortools.sat.IntVar;
 import com.google.ortools.sat.IntervalVar;
@@ -20,11 +21,8 @@ import com.google.ortools.sat.Literal;
 
 /** Code sample to demonstrates how to build an optional interval. */
 public class OptionalIntervalSampleSat {
-  static {
-    System.loadLibrary("jniortools");
-  }
-
   public static void main(String[] args) throws Exception {
+    Loader.loadNativeLibraries();
     CpModel model = new CpModel();
     int horizon = 100;
     IntVar startVar = model.newIntVar(0, horizon, "start");