FMT warning when not found

[Kikadass]

Describe the bug
When linking the library and fmt is not installed a warning appears:

To Reproduce
[Steps to reproduce the behavior:]

1. Link the library with cmake.

In my case I am adding sophus as a third party library with FetchContent:

include(FetchContent)
FetchContent_Declare(
  sophus_install
  URL https://github.com/strasdat/Sophus/archive/refs/tags/1.22.10.tar.gz
)
FetchContent_MakeAvailable(sophus_install)

Expected behavior
fmt is not required, therefor, it should find package in QUIET mode.
find_package(fmt QUIET)

Sophus version: 1.22.10

[strasdat]

This should be fixed by #538, since it removes the fmt dependency.