* Header files that are used by a compilation must be declared

explicitly.  If you forget a dependency, it's simply not visible to
  the compiler, and so the compilation fails.  This is a big plus over
  conventional Make.

examples/default.nix

@@ -0,0 +1,3 @@
+[ (import ./trivial)
+  (import ./simple-header)
+]

examples/simple-header/default.nix

@@ -0,0 +1,11 @@
+let {
+
+  inherit (import ../../lib) compileC link;
+
+  hello = link {objects = compileC {
+    main = ./hello.c;
+    localIncludes = [ [./hello.h "hello.h"] ];
+  };};
+
+  body = [hello];
+}

examples/simple-header/hello.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+
+#include "hello.h"
+
+int main(int argc, char * * argv)
+{
+    printf("Hello " WHAT "\n");
+    return 0;
+}

examples/simple-header/hello.h

@@ -0,0 +1 @@
+#define WHAT "World"

lib/compile-c.sh

@@ -1,3 +1,18 @@
 . $stdenv/setup
+
+mainName=$(basename $main | cut -c34-)
+ln -s $main $mainName
+
+echo "compiling $mainName..."
+
+localIncludes=($localIncludes)
+n=0
+while test $n -lt ${#localIncludes[*]}; do
+    source=${localIncludes[n]}
+    target=${localIncludes[$((n+1))]}
+    ln -s $source $target
+    n=$((n + 2))
+done
+
 mkdir $out
-gcc -Wall -c $main -o $out/$(basename $main).o
+gcc -Wall -c $mainName -o $out/$mainName.o

lib/default.nix

@@ -2,10 +2,10 @@ rec {
 
   inherit (import /home/eelco/nixpkgs/pkgs/system/i686-linux.nix) stdenv;
 
-  compileC = {main}: stdenv.mkDerivation {
+  compileC = {main, localIncludes ? []}: stdenv.mkDerivation {
     name = "compile-c";
     builder = ./compile-c.sh;
-    inherit main;
+    inherit main localIncludes;
   };
 
   link = {objects}: stdenv.mkDerivation {