diff --git a/pkgs/by-name/if/ifm/package.nix b/pkgs/by-name/if/ifm/package.nix
index 29ece74a0bd3..0b8d6e7ccbd9 100644
--- a/pkgs/by-name/if/ifm/package.nix
+++ b/pkgs/by-name/if/ifm/package.nix
@@ -38,7 +38,8 @@ stdenv.mkDerivation (finalAttrs: {
   # Workaround build failure on -fno-common toolchains:
   #   ld: libvars.a(vars-freeze-lex.o):src/libvars/vars-freeze-lex.l:23:
   #     multiple definition of `line_number'; ifm-main.o:src/ifm-main.c:46: first defined here
-  env.NIX_CFLAGS_COMPILE = "-fcommon";
+  # gnu17: libvars uses K&R-style () prototypes; C23 redefines them as (void).
+  env.NIX_CFLAGS_COMPILE = "-fcommon -std=gnu17";
 
   enableParallelBuilding = false; # ifm-scan.l:16:10: fatal error: ifm-parse.h: No such file or directory