Fix the build on Linux amd64/gcc

Without this fix, "make" in the dyncall-0.6 directory would copy the .a
files into a build_out/*/ dir, and thus the nqp ops compilation failed.

I hope this doesn't break the build on other platforms.

Configure.pl

@@ -116,7 +116,7 @@
     }
     else {
         system_or_die('cd 3rdparty/dyncall-0.6 && sh configure');
-        $config{'dyncall_build'} = "cd 3rdparty/dyncall-0.6 && $make";
+        $config{'dyncall_build'} = "cd 3rdparty/dyncall-0.6 && BUILD_DIR=. $make";
     }
 
     fill_template_file('tools/build/Makefile.in', 'Makefile', %config);