Advertise Webp support in desktop file.

[N-R-K]

See: xyb3rt/sxiv#411

[eylles]

i was thinking of adding a macro on the makefile for the full webp support pr, so that if nsxiv isn't compiled with webp support then it won't be advertised on the .desktop file

[N-R-K]

Sxiv doesn't have webp compile option, imlib2 does.

[eylles]

so we leave undoing this to the mantainers of the distros where imlib2 isn't compiled with webp support?

[XPhyro]

Is there a simple way to check if imlib2 has WebP support? If so we could incorporate this in #19.

[TAAPArthur]

Is there a simple way to check if imlib2 has WebP support?

We could just have a HAS_WEBP flag for now that builders can manually set. If set to 1, then we add webp to the desktop file. Off hand I don't know of a portable way to check imlib2 compile objects, but there should be plenty of package manager specific solutions like checking the dependencies of imlib2.
Some ideas (that aren't imlib2 specific in anyway0:

1. Let packages decide
2. We could could check if libwebp is installed but this would yield a false negative if it was statically compiled and no longer on the system.
3. Similar to 2 with the same downside is using ldd on the dynamic lib to see if it is linked to libwebp.
4. Actually try to open a webp image.

Also if the desktop file isn't going to be completely static, may I suggest having a template and then creating and installing the real via the Makefile

[N-R-K]

Is there a simple way to check if imlib2 has WebP support? If so we could incorporate this in #19.

I think we should leave this upto the distro maintainers. And in the case of a source based distro, the user.