* Properly require WebKit for the EPub backend, instead of it being an
  optional dependency.
* Merge EPub tests together to simplify things.
* Make MathJax optional, as it always has been. If missing, MathML will
  likely not display properly, but it's probably not that big of a deal
  as it's not the only use case for the EPub backend. If not found at
  setup time, use the historical default as fallback, in case it gets
  installed later.

Note that all this MathJax stuff is currently unused, because the code
has not been adjusted to make use of this check's results.

This allows enabling the backend automatically depending on whether the
dependencies are satisfied or not, like Autotool's counterpart.

By default, the PDF backend is force-enabled and the Pixbuf backend is
disabled, as with Autotools.

pkg-config input files set -latrilview etc. but the library outputs
created were libview.so, so any project that used pkg-config was broken,
not to mention that the file itself also got renamed from its previous
value and pkg-config wouldn't find it anymore.

Signed-off-by: Oz Tiram <oz.tiram@gmail.com>

Signed-off-by: Oz Tiram <oz.tiram@gmail.com>

Mostly copy and paste from linuxmint/xreader

Signed-off-by: Oz Tiram <oz.tiram@gmail.com>