Stop 'configure --enable-threading' if std::thread doesn't work

Stop 'configure --enable-threading' if std::thread doesn't work

Currently, if you configure gdb with explicit --enable-threading, but
then configure detects std::thread does not work, configure silently
disables threading support and continues configuring.

This patch makes that scenario cause a configuration error, like so:

 $ /home/pedro/gdb/src/configure --enable-threading && make
 ...
 configure: error: std::thread does not work; disable threading
 make[1]: *** [Makefile:11225: configure-gdbsupport] Error 1
 make[1]: Leaving directory '/home/pedro/gdb/build-windows-threads'
 make: *** [Makefile:1041: all] Error 2
 $

Additionally, if you don't explicitly pass --enable-threading, and
std::thread does not work, we will now get a warning (and the build
continues):

 $ /home/pedro/gdb/src/configure && make
 ...
 configure: WARNING: std::thread does not work; disabling threading
 ...

This is similar to how we handle --enable-tui and missing curses.  The
code and error/warning messages were borrowed from there.

Change-Id: I73a8b580d1e2a796b23136920c0e181408ae1b22
Approved-By: Tom Tromey <tom@tromey.com>