Check executable as a fallback

DistributionRequirement should check the executable as a fallback. This is a good idea because it helps for distros that are:

• Does not have the package which results in the user being charge of compilation and installation
• Does not have a package manager (e.g: Slackware, CRUX (I'm aware that CRUX have a package manager, but some people just compile it slackware-style), LFS, some odd version of BSD, Windows, macOS without brew, etc)
• Where it's normal to compile everything (not counting Gentoo, because it has Portage)

Sure, you can argue that you can install a "package manager" (prt-get, slapt-get, brew, pkg, Chocolate, etc) but it's not a part of the distribution or OS and it's not the "right way" to do on the OS.