CMake error when building with external spdlog

[panicgh]

When building with -DKOMPUTE_OPT_USE_SPDLOG:BOOL=TRUE -DKOMPUTE_OPT_USE_BUILT_IN_SPDLOG:BOOL=FALSE cmake outputs the error:

CMake Error at src/logger/CMakeLists.txt:59 (target_compile_definitions):
   Cannot specify compile definitions for target "spdlog" which is not built
   by this project.

because spdlog in https://github.com/KomputeProject/kompute/blob/187b84bd66b86ab64d26aa536d76f11f8c33fa1e/src/logger/CMakeLists.txt#L59C1-L59C111 is an external target, for which definitions cannot be set.

[axsaucedo]

@panicgh the aim is to enable users that may have spdlog set up themselves to work, ie if you import spdlog in your cmakefile which would be external as opposed to using the one built-in