Makefile.am: Protect placeholders for sed

Makefile.am: Protect placeholders for sed

If the placeholder has exactly the same name as the variable used in
autoconf it would be replaced as well. In the case of
“@ENABLE_SPLIT_QUERY@” this led to the sed command “s#False#False#g”.
Inserting additional quotes after the first @ sign prevents this.

Since ENABLE_SPLIT_QUERY was not actually used it is removed.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>