*: fix builds by disregarding warning from new glibc

Also, in case of collectd, the -lgcc_s shouldn't be needed anymore,
as the library is in ${glibc}/lib/ now, which is practically always on RPATH.

In case of seyren it was some stdenv change uncovering the mistake of
putting src into buildInputs.

pkgs/servers/sip/freeswitch/default.nix

@@ -12,6 +12,8 @@ stdenv.mkDerivation rec {
   buildInputs = [ ncurses curl pkgconfig gnutls readline openssl perl libjpeg
     libzrtpcpp ];
 
+  NIX_CFLAGS_COMPILE = "-Wno-error=cpp";
+
   meta = {
     description = "Cross-Platform Scalable FREE Multi-Protocol Soft Switch";
     homepage = http://freeswitch.org/;