Some of them are completely spurious however some (marked FIXME(windows))
may benefit from being looked at.

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.

In order to guarantee that Hammer can build on Windows, an appveyor.yml
and associated build scripts will build hammer and its examples. The idea
is that as soon as the appveyor.yml exists in the repository, pull
requests that would impede Windows portability would be immediately
detected.

The scripts expect CL.EXE to be in the path, and will produce their
results in build/

The highest level of warning is enabled on CL.EXE, minus warnings that
bring CL.EXE to a level that ressembles C99. The only notable warning
that was disabled is the one that tells you about implicit truncating
conversions.

Hammer's source code has quite a few implicit conversions say from a 64bit
unsigned integer to a integer of a lesser size (signed or otherwise)