Use default cpp fmt build script.

vendor/Fmt/test/fuzzing/CMakeLists.txt

@@ -0,0 +1,30 @@
+# Copyright (c) 2019, Paul Dreik
+# License: see LICENSE.rst in the fmt root directory
+
+# Link in the main function. Useful for reproducing, kcov, gdb, afl, valgrind.
+# (Note that libFuzzer can also reproduce, just pass it the files.)
+option(FMT_FUZZ_LINKMAIN "Enables the reproduce mode, instead of libFuzzer" On)
+
+# For oss-fuzz - insert $LIB_FUZZING_ENGINE into the link flags, but only for
+# the fuzz targets, otherwise the CMake configuration step fails.
+set(FMT_FUZZ_LDFLAGS "" CACHE STRING "LDFLAGS for the fuzz targets")
+
+# Adds a binary for reproducing, i.e. no fuzzing, just enables replaying data
+# through the fuzzers.
+function(add_fuzzer source)
+  get_filename_component(basename ${source} NAME_WE)
+  set(name ${basename}-fuzzer)
+  add_executable(${name} ${source} fuzzer-common.h)
+  if (FMT_FUZZ_LINKMAIN)
+    target_sources(${name} PRIVATE main.cc)
+  endif ()
+  target_link_libraries(${name} PRIVATE fmt)
+  if (FMT_FUZZ_LDFLAGS)
+    target_link_libraries(${name} PRIVATE ${FMT_FUZZ_LDFLAGS})
+  endif ()
+  target_compile_features(${name} PRIVATE cxx_generic_lambdas)
+endfunction()
+
+foreach (source chrono-duration.cc float.cc named-arg.cc one-arg.cc two-args.cc)
+  add_fuzzer(${source})
+endforeach ()