*: fix builds by disregarding warning from new glibc

Says: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
CC: #4803. There will likely appear more of these errors on Hydra in time.

pkgs/os-specific/linux/kernel/perf.nix

@@ -25,6 +25,8 @@ stdenv.mkDerivation {
   buildInputs = [ elfutils python perl newt slang pkgconfig] ++
     stdenv.lib.optional withGtk gtk;
 
+  NIX_CFLAGS_COMPILE = "-Wno-error=cpp";
+
   installFlags = "install install-man ASCIIDOC8=1";
 
   inherit elfutils;