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.

---
 configure | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

index 0a19b033bc..9f3777da1b 100755
@@ -296,6 +296,21 @@ 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
+        exit 1
+    fi
 fi
 git="git"
 

2.14.3

Is it worth also mentioning in the error message the existence of scripts/archive-source.sh to help people who need to create tarballs for their test setups (the way we do internally for docker builds)?

thanks
-- PMM