Use fmtlib/fmt as pure header-only dependency

!79 (merged) introduced the use of fmtlib/fmt for string formatting. Since we also use RD/fmt for frequency measurements, this can cause problems for programs that integrate both pmt and RD/fmt, for instance in cudapeak. To solve this, FetchContent_MakeAvailable for fmtlib/fmt is replaced by FetchContent_Populate and an equivalent fmt-header-only target is created.