Make sed in-place more portable

In-place sed (sed -i) works differents for GNU vs BSD version like
what is found on OS X.

This adds a check and does a sed -i for GNU and a sed -i '' for
Darwin only.

This allows my OS X machine to fully compile CM without the need
for any GNU specific packages.

Change-Id: Ic8aada282db9dbe8995aa1fc7b7eebe997a1a5fb