updated all of the compiler flags for the wheezy GCC

There were several new flags that are useful and a few that I've been
waiting for a good time to switch to, so I figured I'd do it all at
once.
diff --git a/aos/build/externals.gyp b/aos/build/externals.gyp
index 0dcabf2..2bfee69 100644
--- a/aos/build/externals.gyp
+++ b/aos/build/externals.gyp
@@ -111,7 +111,9 @@
       'target_name': 'eigen',
       'type': 'none',
       'direct_dependent_settings': {
-        'include_dirs': ['<(externals)/eigen-<(eigen_version)'],
+        'cflags': [
+          '-isystem', '<(externals)/eigen-<(eigen_version)'
+        ],
       },
     },
     {