fix duplicate symbols when using with webp via pods

[mr-slippery-2861]

When build SDWebImage with webp support, it will fail due to "duplicate symbols".

[bpoplauschi]

I tried to repro this, but I get strange compiler errors (like the ones described here #494). It seems those issues were fixed in libwebp, but they never released an update since December 2013, even though they are actively working on the project.

Regarding the issue you are addressing, I can't say if they work or not.

[leafduo]

As discussed in #494, it's a bug in current version of libwebp. libwebp does not support ARM64 correctly, therefore disabling ARM64 support will confirm this pull request works.

[bpoplauschi]

I removed the arm64 support and everything compiled fine, without your fix. I'm not sure I follow

[leafduo]

Did you install webp support via CocoaPods? Like pod 'SDWebImage/WebP'

[bpoplauschi]

Yeap. That's exactly what I did.

platform :ios, '6.1'
pod 'SDWebImage/WebP', :head

[leafduo]

OK, I find the reason.

We have -all_load set in other linker flags.

[leafduo]

Any update?

[bpoplauschi]

I will try with the -all_load flag today or tomorrow. Stay tuned :)

[bpoplauschi]

Hey @leafduo, I just tried with the -all_load flag and I do get the duplicate symbols error. With the fix from @xlChen, everything is ok. I will merge this fix. Thank you both for the support.