Do not override the default installation path to `/usr`

[umlaeute]

on un*x-systems, cmake --install will install to /usr by default:

CMake/Common.cmake

Line 60 in f565154

set(CMAKE_INSTALL_PREFIX "/usr" CACHE PATH

i think this is really harmful, as the expected place is /usr/local, and on typical Linux distributions /usr is reserved for package-manager-installed software.

Also, there might be different permissions setup of installing to /usr rather than to /usr/local.
E.g. on macOS, a simplecmake --install (of Servus, which uses the Eyescale/CMake snippets) fails, because we cannot just install to /usr/share (even when run with sudo)