Tuning malloc.c

[Steve Munroe]

For 64-bit systems it would be nice to adjust some of the malloc tunning 
parameters. For example:

MMAP_AS_MORECORE_SIZE
DEFAULT_TRIM_THRESHOLD
DEFAULT_MMAP_THRESHOLD
HEAP_MIN_SIZE
HEAP_MAX_SIZE

For a 64-bit machine these values should be larger to avoid excessive 
mmap/unmap activity for large transient mallocs. 

Unfortunately there seems to be no clear, arch specific, mechanism for 
overriding the defaults in malloc.c. Should there be a mallocconf.h?