More (?) steps toward jemalloc within Cygwin DLL

[Mark Geisert]

I'm just putting a flag down on this new (to me) territory.  If somebody 
else has claimed this project already, let me know and I'll shove off.

It wasn't much trouble to build a jemalloc.lib and statically link it into 
the Cygwin DLL when the latter is built.  I'm still learning which 
jemalloc configure options are required in order to get complete test 
coverage and to initialize properly within cygwin1.dll.

I'm currently using the "supply your own malloc" mechanism provided by 
Cygwin's malloc_wrapper.cc to overlay the usual dlmalloc-sourced functions 
with replacements from jemalloc.  I suspect there will be allocation 
collisions ahead...

One question has popped up though.  I see from the docs that if one wants 
jemalloc to run thread-aware, it wraps pthread_create() to find out when 
the app has gone from single-threaded to multi-threaded.  But in Cygwin's 
case we'll additionally need to consider cygthreads, won't we?
Thanks,

..mark