freebsd: set BOOTSTRAPPING when building for Linux

FreeBSD loves to access system headers for platform-specific builtins.
This is fine on FreeBSD but often breaks when building utilities for
Linux. `BOOTSTRAPPING` disables using most of these headers.

Instead of adding workarounds to every package, just set `BOOTSTRAPPING`
when the host platform is not FreeBSD.

pkgs/os-specific/bsd/freebsd/pkgs/libelf.nix

@@ -23,6 +23,4 @@ mkDerivation {
     install
     m4
   ];
-
-  BOOTSTRAPPING = !stdenv.hostPlatform.isFreeBSD;
 }

pkgs/os-specific/bsd/freebsd/pkgs/localedef.nix

@@ -35,5 +35,4 @@ mkDerivation (
 
     MK_TESTS = "no";
   }
-  // lib.optionalAttrs (!stdenv.hostPlatform.isFreeBSD) { BOOTSTRAPPING = 1; }
 )

pkgs/os-specific/bsd/freebsd/pkgs/mkDerivation.nix

@@ -92,6 +92,7 @@ lib.makeOverridable (
       # Since STRIP in `makeFlags` has to be a flag, not the binary itself
       STRIPBIN = "${stdenv'.cc.bintools.targetPrefix}strip";
     }
+    // lib.optionalAttrs (!stdenv.hostPlatform.isFreeBSD) { BOOTSTRAPPING = true; }
     // lib.optionalAttrs stdenv'.hostPlatform.isDarwin { MKRELRO = "no"; }
     // lib.optionalAttrs (stdenv'.cc.isClang or false) {
       HAVE_LLVM = lib.versions.major (lib.getVersion stdenv'.cc.cc);

pkgs/os-specific/bsd/freebsd/pkgs/mkcsmapper.nix

@@ -13,8 +13,6 @@ mkDerivation {
     "lib/libiconv_modules/mapper_std"
   ];
 
-  BOOTSTRAPPING = !stdenv.hostPlatform.isFreeBSD;
-
   extraNativeBuildInputs = [
     byacc
     flex

pkgs/os-specific/bsd/freebsd/pkgs/mkesdb.nix

@@ -1,5 +1,4 @@
 {
-  stdenv,
   mkDerivation,
   byacc,
   flex,
@@ -10,8 +9,6 @@ mkDerivation {
 
   extraPaths = [ "lib/libc/iconv" ];
 
-  BOOTSTRAPPING = !stdenv.hostPlatform.isFreeBSD;
-
   extraNativeBuildInputs = [
     byacc
     flex