Add support for jemalloc

[tojamrok]

jemalloc handles threaded allocations better than the default glibc's ptmalloc2, but it's less relevant for Pike. Smaller memory fragmentation is more tangible.

You need jemalloc library available on your OS to be able to compile Pike with jemalloc. Build it with make CONFIGUREARGS="--enable-jemalloc".

The only externally visible change is in _memory_usage() function which depends on malloc's implementation-specific details. glibc's malloc provides struct mallinfo mallinfo(), while jemalloc provides int mallctl(const char *name, void *oldp, size_t *oldlenp, void *newp, size_t newlen); where name argument starts with "stats." (see man jemalloc).

[PeterBortas]

This has been sitting uncommented since forever. Minireview in case you still want this in:

• There is unrelated fiddling with variable names in HTTPLoop (unless that's some sort of namespace pollution from the allocator)
• It's rewriting macros in builtin.cmod just for the sake of it as far as I can see

I don't see why it couldn't be added at least for a while so it would be easier to benchmark between allocators, but the commit should be as minimal as possible.

[tojamrok]

Regarding both comments about src/modules/HTTPLoop/requestobject.c and src/bultin.cmod: both changes are needed here because the code clashes with <stdbool.h>. It's funny that jemalloc includes <stdbool.h> even if it doesn't need C99 flavor to compile.

In <stdbool.h> there's #define true 1. Thus in src/modules/HTTPLoop/requestobject.c change from true to true_val is needed.

In src/builtin.cmod the GET_VAL(NAME) macro is called as GET_VAL (true) which translates to PIKE_CONCAT (get_val_, 1) inside the GET_VAL(NAME) macro which in turn translates to get_val_##1 but other code wants to call get_val_true. The same about TOSTR(true) which translates to #1 but we'd rather have #true instead.

What I'm more worried about is that the commit is too small. There are places where memory is allocated outside of malloc which could clash with jemalloc in unforeseen ways.