Need reviewers : 6875240: Reduce Makefile build time

[Kelly O'Hair]

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

Current webrev:
   http://cr.openjdk.java.net/~ohair/openjdk7/jdk7-build-cygwin-6875240/webrev/

Effectively, these changes use the environment variables as a cache
so that make variables are not repeatedly calculated, avoiding unnecessary
fork/execs. My implementation requires a switch to GNU make 3.81.
(My GNU make 3.81 binaries are at http://cr.openjdk.java.net/~ohair/MakeBinaries/)

I measured a build on Solaris X86 just now, the fork system calls
(forkx and vforkx) went from roughly 113,000 to 50,000.
The build time on Solaris wasn't changed a lot, maybe shaved a few
minutes off, but this should help with cygwin a great deal.
No numbers on cygwin builds yet.

-kto