sysprof-6.pc required at build time for profiler support just for a datadir variable
In Gentoo we provide sysprof-capture-4.pc in a separate tiny package, while sysprof-4.pc/sysprof-6.pc is in the main sysprof package. Users typically install packages by compiling them on the system, as this is Gentoo.
We provide sysprof marks support from mutter by default, but due to this pkg-config dependency, users would have to install the full sysprof package then and they are likely to explicitly disable it then via Gentoo USE flags mechanisms (that end up just passing `-Dprofiler=false`) - often for all packages, not just mutter. This then isn't nice for when they eventually do want to use sysprof down the line.
If I would have a way to just teach the build where the sysprof datadir should be via a meson_option, I could just hardcode that to the expected place, not build-depend on the full sysprof package and default enable the tracing support without drawbacks, as sysprof-capture is tiny. That, or perhaps it would be OK to just fallback to sysprof-capture-4.pc
provided datadir variable instead of `sysprof-<ver>.pc` when the latter isn't found?