- Separate SDK checking from version checking and
make messages clearer.
- Add explicit source & target versions for javac to
make things clearer.
- Rename flag from EXPERIMENTAL_USE_JAVA7_OPENJDK to
EXPERIMENTAL_USE_JAVA7.
- Allow Oracle JDK 1.7 to be used on Mac OS, since there's
no official OpenJDK support for that platform.
Change-Id: I454d2c917ed78f03ec7559a99659fefe7e7d50f3