grow_heap: static linking warns about madvise() not implemented

[Samuel Thibault]

When statically linking on a system on which madvise() wasn't ported,
the linker complains:

/bin/../lib/gcc/i486-gnu/4.0.4/../../../libcrt.a(malloc.o): In function `grow_heap':
(.text+0x3t4): warning: warning: madvise is not implemented and will always fail

should there really be such linker warning for madvise() whose failure
doesn't have any semantic consequences, just no boost of performance?

Samuel