Recent mock packages don't install compilers anymore by default (?), so now we
need to depend on it...

Now that Firefox ESR 60 has been released, most distributions have
dropped ESR 52 for which the older XPC-based add-on was required.

The sole exception to this rule is RHEL6. One might wonder whether it's
worth keeping a package for a distribution that is that ancient; but
since it costs us next to nogthing, might as well keep the XPC extension
there.

Everywhere else, drop it and switch to the webextension.

…ed number

- Deduplicate the "log.h" filename, otherwise we can't correctly use it
  from some locations if we need to (this will break some builds...)
- Add a function to get a configuration value from the config file when
  using C code
- Add a function to figure out the current log level when using C code.

This will probably break the Windows and OSX builds; build system
updates will be necessary.

With that, PKCS#11 logging will work under Linux, even when we didn't
define _DEBUG or DEBUG.

And for keping debug and release behaviour similar, lets not use different init values for the debug level (in case the debug config is missing)

…nd set by VS by default for debug builds

We use DEBUG elsewhere, and _DEBUG here, for essentially the same thing.
That makes no sense. Fix.

The condition ((a != x) || (a != y)) is always true (it can't be equal
to two values at the same time). We actually meant to check if it is
equal to either of them, rather than to be not equal to both.

Coverity CID#191139

…nd them

…ller installer in order to have a signed wrapper around our uninstall script)