configure: add sanity check to catch builds from "git archive"

The "git archive" feature creates tarballs which are missing all
submodule content. GitHub unhelpfully provides users with "Download"
links that claim to give them valid source release tarballs. These
GitHub archives will not be buildable as they are created by the
"git archive" feature and so are missing content. The user gets
unhelpful messages from make such as:

  fatal error: ui/input-keymap-atset1-to-qcode.c: No such file or directory

By adding a sanity check we can give users an informative message about
what they've done wrong.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180418171151.5263-1-berrange@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -300,6 +300,24 @@ then
 else
     git_update=no
     git_submodules=""
+
+    if ! test -f "$source_path/ui/keycodemapdb/README"
+    then
+        echo
+        echo "ERROR: missing file $source_path/ui/keycodemapdb/README"
+        echo
+        echo "This is not a GIT checkout but module content appears to"
+        echo "be missing. Do not use 'git archive' or GitHub download links"
+        echo "to acquire QEMU source archives. Non-GIT builds are only"
+        echo "supported with source archives linked from:"
+        echo
+        echo "  https://www.qemu.org/download/"
+        echo
+        echo "Developers working with GIT can use scripts/archive-source.sh"
+        echo "if they need to create valid source archives."
+        echo
+        exit 1
+    fi
 fi
 git="git"