configure: Don't fall back to gthread coroutine backend

This is a back port of 7c2acc7062fe863cb71ff5849bb121deafe8df4b to the
1.4 stable branch without needing the new error_exit() function.

configure: Don't fall back to gthread coroutine backend

The gthread coroutine backend is broken and does not produce a working
QEMU; it is only useful for some very limited debugging situations.
Clean up the backend selection logic in configure so that it now runs
"if on windows use windows; else prefer ucontext; else sigaltstack".

To do this we refactor the configure code to separate out "test
whether we have a working ucontext", "pick a default if user didn't
specify" and "validate that user didn't specify something invalid",
rather than having all three of these run together. We also simplify
the Makefile logic so it just links in the backend the configure
script selects.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1365419487-19867-3-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>