We wrap all remaining references to __attribute__ into a macro call to
H_GCC_ATTRIBUTE, to allow compilation with MSVC CL.EXE

We substitute H_GCC_ATTRIBUTE to any occurence of __attribute__ and
disable its function with non gcc, non clang compilers.

We use a H_ macro so as to not pollute users' namespaces.

On windows build without c4201 so as to allow anonymous struct. This
shows hammer is not C99 compliant but rather C11.

The pretty printing format chosen is USER:<token name>

We use the registry function `h_get_token_type_name` which returns 0
when no name has been defined. This will print (null) as a token name.

NEWS:
* Switching endianness mid-byte no longer potentially re-reads bytes.
* bit_offset now consistently refers to the number of bits already
  read.
* HParsedTokens now have a bit_length field; this is a size_t.  This
  may be removed for memory reasons.

The bit writer has not yet been updated to match; the result of
switching bit writer endianness in the middle of a byte remains
undefined.

I've found this especially useful in combination with my own
_attr_uint_const for things like flags and type specifiers.

It's possible that its usefulness might be diminished significantly if
there were a built-in bitfield constant parser -- that certainly would
eliminate all of my current uses of it -- but it still seems nicely
symmetric with H_ACT_APPLY.

Tabs after the first non-tab character are crazymaking. I picked 8
because it wasn't 7 and caused the backslashes to line up on
H_ACT_APPLY.

They apparently removed alloc_size at some point (it was a no-op
beforehand), causing the attribute to throw an error when clang compiles
anything including allocator.h.

It's causing unreferenced-variable warnings, and isn't referenced
anywhere aside from benchmark.c. If client code is likely to reference
it, perhaps move it into another header, so people who include hammer.h
don't have to refer to it to have warning-free code.

NetBSD's character class test macros are defined such that they produce
a compiler warning when given a char argument. Adding a cast to the call
fixes this.

CLOCK_THREAD_CPUTIME_ID is a linux-specific timer. On NetBSD, the best
way to get timer information appears to be getrusage, which happens to
be fairly cross-platform.