libspf2: fix darwin build (#353188)

Co-authored-by: Matteo Pacini <[email protected]>

Author: matteo-pacini

pkgs/development/libraries/libspf2/default.nix

@@ -23,6 +23,8 @@ stdenv.mkDerivation rec {
       -e '/bin_PROGRAMS/s/spf_example_static//' src/spf_example/Makefile.am
   '';
 
+  CFLAGS = lib.optionalString stdenv.cc.isClang "-Wno-implicit-function-declaration";
+
   doCheck = true;
 
   meta = with lib; {