Install to the libwpe backends directory

[aperezdc]

Use pkg-config to obtain the value of the backendsdir variable to use the value as location for installing the backend. This also changes the pkg-config .pc file to have that list that directory in the Libs variable, so pplications which need to link into libWPEBackend-fdo-1.0 will still be able to find it.

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This needs the changes from https://github.com/WebPlatformForEmbedded/libwpe/pull/64 and therefore the CI is expected to fail. See https://github.com/WebPlatformForEmbedded/libwpe/issues/59 for more details.