That's a pretty long line, I suggest you wrap it like so:

!IF "$(USE_SSL)"=="true" && "$(USE_WINSSL)"=="true" \
 || "$(USE_SSL)"=="true" && "$(USE_MBEDTLS)"=="true" \
 || "$(USE_MBEDTLS)"=="true" && "$(USE_WINSSL)"=="true"

Otherwise well done!

AFAIK, line wrapping like that does not work in Visual C makefiles

BTW: the parentheses around the && pairs are once again gone in your suggestion. They really should be there.

I haven't checked the docs whether it is supposed to work with preprocessor directives, but it is supported for commands as documented here and I used it in practice without encountering any issues.

BTW: the parentheses around the && pairs are once again gone in your suggestion. They really should be there.

No they are unnecessary, because of a feature called operator precedence which is documented here.

The parentheses may be unnecessary, but for readability I would still leave them there.

The backslash followed by a line-end was indeed documented for commands I see now, even for VC9. But line-ends and Windows are messy stuff, so why not choose the safe solution? Better safe than sorry.

Fine with me. Priority is that the building works.

I (although I like to say we so that it doesn't sound so much like a dictatorship), like code that isn't terribly wide as it makes it easier to read so I'm for breaking the line up into several with backslash-continuations.

With or without the parentheses? If you break it up the logic is clearer, but nevertheless...

I don't have a strong opinion on that but would leave that to you who're writing this, but my general preference is to add parentheses to help readers to not have to dig out any forgotten precedence rules to understand the order.

Added this to the PR. Please merge.