jemalloc-sys always builds jemalloc with stats enabled regardless of the stats cargo feature

[rocallahan]

The jemalloc checkout included in jemalloc-sys has, in configure.ac:

dnl Enable statistics calculation by default.
AC_ARG_ENABLE([stats],
  [AS_HELP_STRING([--disable-stats],
                  [Disable statistics calculation/reporting])],
[if test "x$enable_stats" = "xno" ; then
  enable_stats="0"
else
  enable_stats="1"
fi
],
[enable_stats="1"]
)
if test "x$enable_stats" = "x1" ; then
  AC_DEFINE([JEMALLOC_STATS], [ ])
fi
AC_SUBST([enable_stats])

but jemalloc-sys build.rs has

    if env::var("CARGO_FEATURE_STATS").is_ok() {
        info!("CARGO_FEATURE_STATS set");
        cmd.arg("--enable-stats");
    }

So stats are always enabled in the jemalloc build.

[rocallahan]

Seems unfortunate that every Rust user of jemalloc has stats enabled when that is not the intended default. I'll send a PR.