Added ndk compilation

Author: lev-zakaryan

opuslib/build.gradle

@@ -34,6 +34,23 @@ android {
             withJavadocJar()
         }
     }
+
+        sourceSets.main {
+        jni.srcDirs = []                // This prevents the auto generation of Android.mk
+        jniLibs.srcDir 'src/main/libs'  // This is not necessary unless you have precompiled libraries in your project.
+    }
+
+    task ndkBuild(type: Exec) {
+        //if (Os.isFamily(Os.FAMILY_WINDOWS)) {
+            commandLine 'ndk-build.cmd', '-C', file('src/main/jni').absolutePath
+        //} else {
+        //    commandLine 'ndk-build', '-C', file('src/main/jni').absolutePath
+        //}
+    }
+
+    tasks.withType(JavaCompile) {
+        compileTask -> compileTask.dependsOn ndkBuild
+    }
 }
 
 dependencies {