download_and_compile.sh: improve user-interaction

Since the prompt is repeated by _yes_no_quit_prompt() every time an
invalid answer is given, a short sentence is better than the empty
string.

download_and_compile.sh

@@ -254,7 +254,7 @@ FGData (it is\nmaintained by FlightGear developers). Answer 'y' to proceed \
 this way. If you\nanswer 'n', we'll *try* to clone FGData from SourceForge \
 using https. Answer 'q'\nif you want to quit. "
     fi
-    _yes_no_quit_prompt "" y; prompt_res=$?
+    _yes_no_quit_prompt "What is your choice?" y; prompt_res=$?
     set -e
     case $prompt_res in
         0) retcode=0 ;;