Try to protect RHEL 8 from whatever was happening in its build system to break stuff.

RHEL 8 does not, apparently, specify source and build directories when it calls `cmake` (not `cmake --build`, that's a different step). This causes CMake to set the source and build directories to both be the current directory; apparently this causes the macros `%_vpath_srcdir` and `%_vpath_builddir` to be ignored during `cmake --build`, which may be the ultimate cause of the issues we've encountered recently with the CMake build system on RHEL 8 builders.

We'll see how this commit affects the RHEL 8 builders. It shouldn't affect the others, because when the builder is not RHEL 8 the macro `%sourceAndBuilddir` should expand to an empty string.

This may or may not work. RPM syntax is a little confusing (beyond the basics).

While connected to a COPR VM and chrooted into an EPEL 8 environment, it appears that the EPEL 8 build environment has a CMake which:
1. Isn't directed to create an out of source build, for whatever reason (I had troubles determining exactly what was going on)
2. Assumes that the current directory is both the source and build directory (the current directory being the source directory which is changed to by rpmbuild automatically, I think).

As far as I can tell, the easiest way around issue MesserLab#440 to enable MesserLab#465 and MesserLab#468 is to make the build directory and change to it before calling cmake --build, but this only works if we manually specify the source and build directory when calling `%cmake`, which shouldn't be too delicate, as long as the definition of `%cmake` doesn't include source and build directories on other distributions.

I'm out of practice with rpmbuild and COPR's finer details, or I never was practiced in using these at this level, and I don't know a better way of debugging the build process at this point after investigating through SSH than to just submit a new build. I got stuck trying to create a new SRPM with the edited spec file. It's easier to just edit on GitHub like this and have an automatic build tell me if things are going to work as I expect this might. If it doesn't then I'm calling in reinforcements to investigate the issue for me and offer their support (the Fedora community!).

Big sad

https://docs.fedoraproject.org/en-US/packaging-guidelines/RPMMacros/#_macros_set_for_the_rpm_and_srpm_build_process