Exclude -DDEBUG from the libsass build

If compiling using the debug build of node
-DDEBUG and -D_DEBUG flags are automatically
added.

This causes libsass to produce additional
debugging output that may interfere with
tests.

Once this is resolved, this can be reverted.

Reported-as: sass/libsass#1518

src/libsass.gyp

@@ -6,6 +6,9 @@
       'defines': [
          'LIBSASS_VERSION="<!(node -e "process.stdout.write(require(\'../package.json\').libsass)")"'
       ],
+      'defines!': [
+         'DEBUG'
+      ],
       'sources': [
         'libsass/ast.cpp',
         'libsass/base64vlq.cpp',