Update to dlmalloc 2.8.6 and set HAVE_MMAP to 0 only if compiling under EMSCRIPTEN to fix an allocator failure issue on OSX when building test_dlmalloc to native code. Fixes #730.
Skip, or simulate success for the test_dlmalloc test that checks memory allocation linearity/contiguity when compiling it natively to OSX. On that platform, this condition has been detected to not hold. Retain the check still for other platforms.
Merge pull request #846 from juj/dlmalloc_286

Dlmalloc 286