Status of ccache

[Dan Smith]

JDK-8027584 disabled ccache by default, I gather because it doesn't work in Cygwin, and secondarily because of vague general problems with it.

The documentation (README-builds.html) still unambiguously endorses it, although doesn't say anything about how to turn it on (configure with --enable-ccache).

I'm curious about the current state of affairs, and suggest updating the documentation to reflect the answer.

—Dan