treewide: Make explicit that 'dev' output of libevent is used

pkgs/applications/networking/cluster/mesos/default.nix

@@ -104,7 +104,7 @@ in stdenv.mkDerivation rec {
     "--enable-ssl"
     "--with-ssl=${openssl}"
     "--enable-libevent"
-    "--with-libevent=${libevent}"
+    "--with-libevent=${libevent.dev}"
   ] ++ lib.optionals stdenv.isLinux [
     "--with-network-isolator"
   ];

pkgs/development/libraries/coprthr/default.nix

@@ -21,7 +21,7 @@ stdenv.mkDerivation rec {
 
   configureFlags =
     [ "--with-libelf=${libelf}"
-      "--with-libevent=${libevent}"
+      "--with-libevent=${libevent.dev}"
       "--with-libconfig=${libconfig}"
       "--with-opencl-icd-path=$out/etc/OpenCL/vendors"
       "--enable-user-install"

pkgs/servers/mail/opensmtpd/extras.nix

@@ -17,7 +17,7 @@ stdenv.mkDerivation rec {
     "--sysconfdir=/etc"
     "--localstatedir=/var"
     "--with-privsep-user=smtpd"
-    "--with-libevent-dir=${libevent}"
+    "--with-libevent-dir=${libevent.dev}"
 
     "--with-filter-clamav"
     "--with-filter-dkim-signer"