Check_os_dependency doesn't support Gentoo, nor custom sysroots

[casparvl]

Issue
While trying to build with a custom sysroot based on Gentoo-prefix, I found that my OpenMPI installation failed. The OpenMPI EasyBlock picked up on the verbs installation of the host OS, but with a custom sysroot, one would want it to check the custom sysroot for installed packages, not the host.

Solution
Two components are needed here:

• check_os_dependency now relies on get_os_name() to determine which package manager to query for the availability of the package. That behaviour should be changed if a custom sysroot is defined, e.g. we could make a config item for EasyBuild in which one can specify the package mananger for the custom prefix.
• Support for Gentoo package management should be included.