Use pom property to specify Meson builddir

It's cleaner for the outer script to set builddir equally for Meson and
Maven, so parameterize.

Author: marktsuchida

MMCoreJ_wrap/pom.xml

@@ -48,6 +48,7 @@
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <meson.build.dir>builddir</meson.build.dir>
     </properties>
 
     <build>
@@ -89,7 +90,7 @@
                         </goals>
                         <configuration>
                             <sources>
-                                <source>builddir/generated-java</source>
+                                <source>${meson.build.dir}/generated-java</source>
                             </sources>
                         </configuration>
                     </execution>