Use __attribute__ when building with Clang on Windows

[bjosv]

Since Clang supports __attribute__ we can avoid disabling it and use packed sdshdrX structs when building with Clang on Windows. This also make sure we don't affect subsequent header files that require __attribute__.

Note:
Clang attempts to be a drop-in replacement for MSVC and therefor defines _MSC_VER to be able to use existing libraries/headers.

Fixes #1042

[bjosv]

As I understand it we don't want to change to much in sds.c/.h, but this is an option to fix #1042

[michael-grunder]

Clang attempts to be a drop-in replacement for MSVC and therefor defines _MSC_VER

It defines __GNUC__ as well which I recently learned the hard way. 😄

As I understand it we don't want to change to much in sds.c/.h, but this is an option to fix #1042

We may actually have more leeway here because we did a mass renaming of sds symbols to hi_<symbol> when Redis last upgraded the version of hiredis it builds.

I'll get this merged.

[bjosv]

It defines __GNUC__ as well which I recently learned the hard way. smile

Ouch :)
It's a bit funny that one can find stackoverflow post like How to tell Clang to stop pretending to be other compilers?

[Danvil]

Thank you for fixing the issue!