configure: Don't claim 'unsupported host OS' when better message available

The change in commit 898be3e041 which made completely
unrecognized OSes cause an error_exit "Unsupported host OS"
has some unfortunate unintended effects:
 * if you run 'configure --help' on an unsupported host OS
   (eg if intending to use it as a build machine for a
   cross compile to a supported host) then the message
   is printed instead of --help
 * if the C compiler doesn't work or is missing (eg if
   you passed an incorrect --cross-prefix by mistake)
   the message is printed instead of the more useful
   'compiler does not exist or does not work' message

Fix this by postponing the error_exit in this situation
until later, when we have already identified the more
useful cases for this.

The long term fix for this would be to move handling
of --help much further up in the configure script,
and make its output not dependent on checks that configure
runs. However for 2.9 this would be too invasive.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Tested-by: Stefan Weil <sw@weilnetz.de>

configure

@@ -323,6 +323,7 @@ replication="yes"
 
 supported_cpu="no"
 supported_os="no"
+bogus_os="no"
 
 # parse CC options first
 for opt do
@@ -694,7 +695,10 @@ Linux)
   supported_os="yes"
 ;;
 *)
-  error_exit "Unsupported host OS $targetos"
+  # This is a fatal error, but don't report it yet, because we
+  # might be going to just print the --help text, or it might
+  # be the result of a missing compiler.
+  bogus_os="yes"
 ;;
 esac
 
@@ -1460,6 +1464,14 @@ if ! compile_prog ; then
     error_exit "\"$cc\" cannot build an executable (is your linker broken?)"
 fi
 
+if test "$bogus_os" = "yes"; then
+    # Now that we know that we're not printing the help and that
+    # the compiler works (so the results of the check_defines we used
+    # to identify the OS are reliable), if we didn't recognize the
+    # host OS we should stop now.
+    error_exit "Unrecognized host OS $targetos"
+fi
+
 # Check that the C++ compiler exists and works with the C compiler
 if has $cxx; then
     cat > $TMPC <<EOF