httpd: Support building with another multi-processing module

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

@@ -2,10 +2,14 @@
 , sslSupport, proxySupport ? true
 , apr, aprutil, pcre
 , ldapSupport ? true, openldap
+, # Multi-processing module to use.  This is built into the server and
+  # cannot be selected at runtime.
+  mpm ? "prefork"
 }:
 
 assert sslSupport -> openssl != null;
 assert ldapSupport -> aprutil.ldapSupport && openldap != null;
+assert mpm == "prefork" || mpm == "worker" || mpm == "event";
 
 stdenv.mkDerivation rec {
   version = "2.2.22";
@@ -24,6 +28,9 @@ stdenv.mkDerivation rec {
   # passing simply CFLAGS did not help, then I go by NIX_CFLAGS_COMPILE
   NIX_CFLAGS_COMPILE = "-iquote ${apr}/include/apr-1";
 
+  # Required for ‘pthread_cancel’.
+  NIX_LDFLAGS = "-lgcc_s";
+
   configureFlags = ''
     --with-z=${zlib}
     --with-pcre=${pcre}
@@ -32,8 +39,11 @@ stdenv.mkDerivation rec {
     ${if proxySupport then "--enable-proxy" else ""}
     ${if sslSupport then "--enable-ssl --with-ssl=${openssl}" else ""}
     ${if ldapSupport then "--enable-ldap --enable-authnz-ldap" else ""}
+    --with-mpm=${mpm}
   '';
 
+  enableParallelBuilding = true;
+
   postInstall = ''
     echo "removing manual"
     rm -rf $out/manual