Fixed autoreconf failure on the jemalloc repo

The autoconf manual states:

"In order to do its job, autoheader needs you to document all of the
symbols that you might use; i.e., there must be at least one AC_DEFINE
or one AC_DEFINE_UNQUOTED call with a third argument for each symbol
(see section 7.1 Defining C Preprocessor Symbols)."

This patch adds empty documentation ([]) to avoid autoheader failures.