diff --git a/pkgs/development/interpreters/s9fes/default.nix b/pkgs/development/interpreters/s9fes/default.nix
index 7cf60b00eb9b..4107b28e1cf9 100644
--- a/pkgs/development/interpreters/s9fes/default.nix
+++ b/pkgs/development/interpreters/s9fes/default.nix
@@ -22,7 +22,11 @@ stdenv.mkDerivation rec {
   '';
 
   buildInputs = [ ncurses ];
+  preBuild = ''
+    makeFlagsArray+=(CFLAGS="-O2 -std=c89")
+  '';
   makeFlags = [ "CC=${stdenv.cc.targetPrefix}cc" "PREFIX=$(out)" ];
+
   enableParallelBuilding = true;
   # ...-bash-5.2-p15/bin/bash: line 1: ...-s9fes-20181205/bin/s9help: No such file or directory
   # make: *** [Makefile:157: install-util] Error 1