Add custom memory allocator to MinGW and MacOS builds

Add custom memory allocator to MinGW and MacOS builds

The standard allocator on Windows is pretty bad prior
to Windows Vista, and nedmalloc is better than the
modified dlmalloc provided with newer versions of the
MinGW libc.

NedMalloc stats in Git
----------------------
  All results are the best result out of 3 runs. The
  benchmarks have been done on different hardware, so
  the repack times are not comparable.

  These benchmarks are all based on 'git repack -adf'
  on the Linux kernel.

  XP
  -----------------------------------------------
  MinGW               Threads  Total Time   Speed
  -----------------------------------------------
  3.4.2                  (1T)  00:12:28.422
  3.4.2     + nedmalloc  (1T)  00:07:25.437 1.68x

  3.4.5                  (1T)  00:12:20.718
  3.4.5     + nedmalloc  (1T)  00:07:24.809 1.67x

  4.3.3-tdm              (1T)  00:12:01.843
  4.3.3-tdm + nedmalloc  (1T)  00:07:16.468 1.65x

  4.3.3-tdm              (2T)  00:07:35.062
  4.3.3-tdm + nedmalloc  (2T)  00:04:57.874 1.54x

  Vista
  -----------------------------------------------
  MinGW               Threads  Total Time   Speed
  -----------------------------------------------
  4.3.3-tdm              (1T)  00:07:40.844
  4.3.3-tdm + nedmalloc  (1T)  00:07:17.548 1.05x

  4.3.3-tdm              (2T)  00:05:33.746
  4.3.3-tdm + nedmalloc  (2T)  00:05:27.334 1.02x

  Mac Mini
  -----------------------------------------------
  GCC                 Threads  Total Time   Speed
  -----------------------------------------------
  i686-darwin9-4.0.1     (2T)  00:09:57.346
  i686-darwin9-4.0.1+ned (2T)  00:08:51.072 1.12x

Signed-off-by: Marius Storm-Olsen <marius@trolltech.com>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>