Move platform headers and tricks from jemalloc_internal.h.in to a new jemalloc_internal_decls.h header
Define INFINITY when it's not defined
Correctly return exit code from thd_join on Windows
Fixup after 3a730df (Avoid pointer arithmetic on void*[...])
STATIC_PAGE_SHIFT for cross-compiling jemalloc

Sets `STATIC_PAGE_SHIFT` for cross-compiling jemalloc to 12. A
shift of 12 represents a page size of 4k for practically all
platforms.
Make sure initialization occurs prior to running tests.
Use nallocx() rather than mallctl() to trigger initialization.

Use nallocx() rather than mallctl() to trigger initialization, because
nallocx() has no side effects other than initialization, whereas
mallctl() does a bunch of internal memory allocation.
Fix -Wsign-compare warnings
Fix -Wsometimes-uninitialized warnings