Make test/cxxflags use a user define instead of __OPTIMIZE__

__OPTIMIZE__ is a gcc-specific define, so this was broken on Windows
after https://code.google.com/p/gyp/source/detail?r=1893.

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/235033003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1895 78cadc50-ecff-11dd-a971-7dbc132099af