Preliminary work on jdk build time improvements

[Kelly O'Hair]

Here is a preliminary (and incomplete) webrev on some changes
I've been working on to reduce the build time.

6875240: Reduce Makefile build time by limiting repeated exec's (mostly for cygwin building)

http://cr.openjdk.java.net/~ohair/openjdk7/jdk7-build-cygwin/webrev/

The Windows cygwin builds will benefit the most on this, but
all platforms will get some benefits.

Comments and suggestions on this are welcome.

-kto