mysql_install_db: assume builddir is dirname0

The assumption in the original commit for --builddir (648d3ced),
was to assume that without a --builddir, and when --srcdir
is specified, that the builddir is the same as the srcdir.

The problem is that this assumption does not hold for out-of-source
builds and we can figure out the builddir by looking for where
mysql_install_db script is.

As mysql_install_db is in the builddir, we use dirname0 as the builddir after
checking that my_print_defaults is also located from dirname0, otherwise
default to old behavior.

scripts/mysql_install_db.sh

@@ -245,8 +245,13 @@ if test -n "$srcdir"
 then
   if test -z "$builddir"
   then
+    if test -x "$dirname0/extra/my_print_defaults"
+    then
+      builddir="$dirname0"
+    else
       builddir="$srcdir"
     fi
+  fi
   print_defaults="$builddir/extra/my_print_defaults"
 elif test -n "$basedir"
 then