UseJava: Add support for javah tool

Add a `create_javah` API.

Tests/JavaJavah/B.cpp

@@ -0,0 +1,10 @@
+
+#include <jni.h>
+#include <stdio.h>
+
+#include "B.h"
+
+JNIEXPORT void JNICALL Java_B_printName(JNIEnv *, jobject)
+{
+  printf("B\n");
+}

Tests/JavaJavah/B.java

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+class B
+{
+  public B()
+    {
+    }
+
+    public native void printName();
+
+    static {
+        try {
+
+            System.loadLibrary("B");
+
+        } catch (UnsatisfiedLinkError e) {
+            System.err.println("Native code library failed to load.\n" + e);
+            System.exit(1);
+        }
+    }
+}

Tests/JavaJavah/CMakeLists.txt

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+project(helloJavah Java CXX)
+
+cmake_minimum_required (VERSION 2.6)
+set(CMAKE_VERBOSE_MAKEFILE 1)
+
+find_package(Java COMPONENTS Development)
+include (UseJava)
+
+# JNI support
+find_package(JNI)
+
+add_jar(hello3 B.java HelloWorld2.java)
+create_javah(TARGET B_javah CLASSES B CLASSPATH hello3)
+
+add_library(B SHARED B.cpp)
+add_dependencies(B B_javah)
+
+target_include_directories(B PRIVATE ${CMAKE_CURRENT_BINARY_DIR}
+                                     ${JAVA_INCLUDE_PATH}
+                                     ${JAVA_INCLUDE_PATH2})

Tests/JavaJavah/HelloWorld2.java

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+class HelloWorld2
+{
+    public static void main(String args[])
+    {
+        B b;
+        b = new B();
+        b.printName();
+        System.out.println("Hello World!");
+    }
+}