MB-12745: Don't attempt to check for both -ljemalloc and path/to/jemalloc.so

On Ubuntu 12.04 (at least) CHECK_LIBRARY_EXISTS(jemalloc) fails when
CMAKE_REQUIRED_LIBRARIES is also set, as the linker complains about
specifying both a -ljemalloc and path/to/jemalloc.so.

Fix this by changing FindCouchbaseJemalloc.cmake to use
CHECK_SYMBOL_EXISTS() instead of CHECK_LIBRARY_EXISTS() which allows
us to just specify a path/to/library.so

Change-Id: Ic763cf1b594e955e6f0b4141eda23c14a475dc05
Reviewed-on: http://review.couchbase.org/43639
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>