php fpm: add systemd support

PHP FPM will now notify systemd when it's done initializing and ready to
serve requests.

Additionally ```systemctl status phpfpm``` will now show statistics such
as:

```
Status: "Processes active: 0, idle: 8, Requests: 0, slow: 0, Traffic: 0req/sec"
```

pkgs/development/interpreters/php/default.nix

@@ -2,7 +2,7 @@
 , mysql, libxml2, readline, zlib, curl, postgresql, gettext
 , openssl, pkgconfig, sqlite, config, libjpeg, libpng, freetype
 , libxslt, libmcrypt, bzip2, icu, openldap, cyrus_sasl, libmhash, freetds
-, uwimap, pam, gmp, apacheHttpd, libiconv }:
+, uwimap, pam, gmp, apacheHttpd, libiconv, systemd }:
 
 let
 
@@ -19,11 +19,12 @@ let
 
       enableParallelBuilding = true;
 
-      buildInputs = [ flex bison pkgconfig ];
+      buildInputs = [ flex bison pkgconfig systemd ];
 
       configureFlags = [
         "EXTENSION_DIR=$(out)/lib/php/extensions"
-      ] ++ lib.optional stdenv.isDarwin "--with-iconv=${libiconv}";
+      ] ++ lib.optional stdenv.isDarwin "--with-iconv=${libiconv}"
+        ++ lib.optional stdenv.isLinux  "--with-fpm-systemd";
 
       flags = {