Silence conversion warning from range-v3 0.10.0

tools/gcc-9.dds

@@ -2,7 +2,8 @@ Compiler-ID: GNU
 C++-Version: C++17
 C-Compiler: gcc-9
 C++-Compiler: g++-9
-Flags: -D SPDLOG_COMPILED_LIB -fconcepts -Werror=return-type
+# Range-v3 0.10.0 contains an accidental conversion warning
+Flags: -D SPDLOG_COMPILED_LIB -fconcepts -Werror=return-type -Wno-conversion
 Link-Flags: -static-libgcc -static-libstdc++
 Debug:  True
 Compiler-Launcher: ccache