configure: Fix error message when C compiler is not working

If you try to run the configure script on a system without a working
C compiler, you get a very misleading error message:

 ERROR: Unrecognized host OS (uname -s reports 'Linux')

Some people already opened bug tickets because of this problem:

 https://gitlab.com/qemu-project/qemu/-/issues/2057
 https://gitlab.com/qemu-project/qemu/-/issues/2288

We should rather tell the user that we were not able to use the C
compiler instead, otherwise they will have a hard time to figure
out what was going wrong.

While we're at it, let's also suppress the "unrecognized host CPU"
message in this case since it is rather misleading than helpful.

Fixes: 264b803721 ("configure: remove compiler sanity check")
Message-ID: <20240513114010.51608-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

@@ -411,7 +411,9 @@ else
   # Using uname is really broken, but it is just a fallback for architectures
   # that are going to use TCI anyway
   cpu=$(uname -m)
-  echo "WARNING: unrecognized host CPU, proceeding with 'uname -m' output '$cpu'"
+  if test "$host_os" != "bogus"; then
+    echo "WARNING: unrecognized host CPU, proceeding with 'uname -m' output '$cpu'"
+  fi
 fi
 
 # Normalise host CPU name to the values used by Meson cross files and in source
@@ -894,6 +896,13 @@ EOF
 exit 0
 fi
 
+# Now that we are sure that the user did not only want to print the --help
+# information, we should double-check that the C compiler really works:
+write_c_skeleton
+if ! compile_object ; then
+    error_exit "C compiler \"$cc\" either does not exist or does not work."
+fi
+
 # Remove old dependency files to make sure that they get properly regenerated
 rm -f ./*/config-devices.mak.d