USE_JEMALLOC in the OSS build if we have jemalloc

USE_JEMALLOC in the OSS build if we have jemalloc

Summary:
Nothing defines USE_JEMALLOC in the OSS build today and that causes
some portability problems.

Specifically, the homebrew recipe will make libjemalloc available and our
configure script will detect it and add it to the library flags.

Our subsequent check for `malloc_usable_size` then finds this function in
libjemalloc.

When later attempting to build wangle against the homebrew folly we get
compilation failures because the prototype for `malloc_usable_size` is only
available in the jemalloc headers and nothing in the saved configuration for
folly is set up for this to be pulled in as it it guarded by `USE_JEMALLOC`.

This attempts to resolve the situation by forcing on `USE_JEMALLOC` when
we detect the library in configure.

This is made a little more complicated because we cannot set `USE_JEMALLOC`
in the OSS build; it gets rewritten to have a `FOLLY_` prefix.  Since we
have code outside of folly that requires that this symbol be `USE_JEMALLOC`,
I've changed the conditional to check for both flavors of the symbol, with
and without the prefix.

Reviewed By: yfeldblum

Differential Revision: D4289176

fbshipit-source-id: 756bc815c3ef1fac454e603feb72155d98c5aadd