Installing SLADE on Linux installs fmt system-wide

[jjbarr]

SLADE Version: 3.2.3+5+g23d86d96
Operating System: Linux

Issue Details:

I attempted to update SLADE using the slade-git AUR package on ArchLinux. When doing so, the install bailed because the install archive contained the entirety of fmt (conflicting with my own install of fmt). This occurred because SLADE was configured to use its own vendored copy of fmt, which is included in full with the install.

The root cause of this seems to be that fmt generates a install target it FMT_INSTALL is set to ON, which it is by default. If this is in fact correct, the solution would be to ensure this is set to OFF in the default build.