Use a custom, NAR-specific assembly descriptor

This assembly descriptor is designed to incorporate all of the
platform-specific build projects available in the current target/nar/
directory.

The result is a .jar file with the -multi-platform.jar suffix that
includes all of the native libraries ready for use with the
native-lib-loader aware NarSystem class.

To use it, the project needs to be built from identical revisions for all
platforms that need to be supported. Then, the

	target/nar/<artifactId>-<version>-<aol>-jni/

directories need to be accumulated on one platform and a final `mvn` run
will produce the desired uber .jar file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

pom.xml

@@ -36,9 +36,9 @@
                             <mainClass>c.hello</mainClass>
                         </manifest>
                     </archive>
-                    <descriptorRefs>
-                        <descriptorRef>jar-with-dependencies</descriptorRef>
-                    </descriptorRefs>
+                    <descriptors>
+                        <descriptor>src/main/assembly/uber.xml</descriptor>
+                    </descriptors>
                 </configuration>
                 <executions>
                     <execution>

src/main/assembly/uber.xml

@@ -0,0 +1,15 @@
+<assembly>
+  <id>multi-platform</id>
+  <formats>
+    <format>jar</format>
+  </formats>
+  <includeBaseDirectory>false</includeBaseDirectory>
+  <dependencySets>
+    <dependencySet>
+      <useProjectAttachments>true</useProjectAttachments>
+      <useTransitiveFiltering>false</useTransitiveFiltering>
+      <outputDirectory>/</outputDirectory>
+      <unpack>true</unpack>
+    </dependencySet>
+  </dependencySets>
+</assembly>