A few more Java7 related clean ups.

- 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
diff --git a/core/combo/javac.mk b/core/combo/javac.mk
index 74490fd..2aea3e1 100644
--- a/core/combo/javac.mk
+++ b/core/combo/javac.mk
@@ -8,10 +8,10 @@
 #   COMMON_JAVAC -- Java compiler command with common arguments
 #
 
-ifeq ($(EXPERIMENTAL_USE_JAVA7_OPENJDK),)
+ifeq ($(EXPERIMENTAL_USE_JAVA7),)
 common_flags := -target 1.5 -Xmaxerrs 9999999
 else
-common_flags := -Xmaxerrs 9999999
+common_flags := -source 1.7 -target 1.7 -Xmaxerrs 9999999
 endif