Make configure always generate a dummy bdb/build_unix/db.h file. Ugh.

configure.in:
  Always make a dummy bdb/build_unix/db.h, because the
  dependencies are included in the Makefile, which gets
  distributed, so this file has to exist.  This is ugly.

configure.in

@@ -1909,12 +1909,16 @@ dnl       echo "bdb = '$bdb'; inc = '$bdb_includes', lib = '$bdb_libs'"
   else
     if test -d bdb; then :
     else
-      mkdir bdb && mkdir bdb/build_unix && cat <<EOF > bdb/build_unix/db.h
+      mkdir bdb && mkdir bdb/build_unix
+    fi
+
+    if test -r bdb/build_unix/db.h; then :
+    else
+      cat <<EOF > bdb/build_unix/db.h
 
 This file is a placeholder to fool make.  The way that automake
-handles 'make distcheck' (calling the VPATH configure with different
-options from the original configure) causes make to think that mysqld
-depends on this file, even though Berkeley DB isn't being used.
+and GNU make work together causes some files to depend on this
+header, even if we're not building with Berkeley DB.
 
 Obviously, if this file *is* used, it'll break and hopefully we can find
 out why this file was generated by $(top_srcdir)/configure instead of