treewide: *inputs are lists

pkgs/servers/http/apache-httpd/2.4.nix

@@ -44,10 +44,6 @@ stdenv.mkDerivation rec {
   # Required for ‘pthread_cancel’.
   NIX_LDFLAGS = stdenv.lib.optionalString (!stdenv.isDarwin) "-lgcc_s";
 
-  preConfigure = ''
-    configureFlags="$configureFlags --includedir=$dev/include"
-  '';
-
   configureFlags = [
     "--with-apr=${apr.dev}"
     "--with-apr-util=${aprutil.dev}"
@@ -60,6 +56,7 @@ stdenv.mkDerivation rec {
     "--enable-cern-meta"
     "--enable-imagemap"
     "--enable-cgi"
+    "--includedir=${placeholder "dev"}/include"
     (stdenv.lib.enableFeature proxySupport "proxy")
     (stdenv.lib.enableFeature sslSupport "ssl")
     (stdenv.lib.withFeatureAs libxml2Support "libxml2" "${libxml2.dev}/include/libxml2")