Fix FontForge compilation without 10.7 SDK

FontForge uses the FlatCarbon headers, which are gone in Mountain Lion.
To fix this, the formula was adjusted to use the 10.7 SDK. However, this
prevents compiling FontForge on a system without the 10.7 SDK. Fix this
by replacing the FlatCarbon imports with the correct imports to the
non-flat headers.

Closes #17909.