secur32: Take schannel backend capabilities into account when configuring enabled protocols.

[Ken Thomases]

Hi Jacek,

On Apr 10, 2013, at 4:24 AM, Jacek Caban wrote:

> On 3/28/13 8:31 PM, Ken Thomases wrote:
>> Mac OS X 10.8 introduced support for TLS 1.1 and 1.2.
> 
> Can someone with Mac OS X 10.8 test the attached patch for me, please. All I need is to verify that it compiles and when running dlls/secur32/tests/secur32_test.exe.so schannel, TLS 1.1 and TLS 1.2 are listed as supported protocol.

Thanks for doing this.  I don't have a 10.8 build system handy, myself, or I would have done it or would test what you've done.

However, Apple's guidance on using weak linking says that you must explicitly compare against NULL.  They don't quite say that testing the symbol as a standalone boolean expression won't work, but they do say that using negation (the ! operator) isn't sufficient.
https://developer.apple.com/library/mac/documentation/developertools/conceptual/cross_development/Using/using.html#//apple_ref/doc/uid/20002000-SW4

I'm not sure what the rationale is, but I think there's a peculiarity of what the compiler thinks it knows versus what the linker actually knows.  Basically, the compiler figures there's a symbol that will be resolved (or cause a link error), so the check can be optimized away somehow.  I think.

Other than that, the patch looks good to me.

-Ken