build: use -fPIC for native addons on UNIX

No -fPIC on 32 bits Linux but do enable for the other UNIX flavors.

tools/addon.gypi

@@ -15,6 +15,9 @@
       }],
       [ 'OS=="win"', {
         'libraries': [ '-l<(node_root_dir)/$(Configuration)/node.lib' ],
+      }],
+      [ 'OS=="freebsd" or OS=="openbsd" or OS=="solaris" or (OS=="linux" and target_arch!="ia32")', {
+        'cflags': [ '-fPIC' ],
       }]
     ]
   }