GN: Only use -fPIC where needed

runtime/bin/BUILD.gn

 # Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 import("gypi_contents.gni")
 import("../runtime_args.gni")
 
 # Generate a resources.cc file for the service isolate without Observatory.
 action("gen_resources_cc") {
   visibility = [ ":*" ]  # Only targets in this file can see this.
@@ skipping 803 matching lines @@
   ]
   sources = [
     "test_extension.c",
     "test_extension_dllmain_win.cc",
   ]
   include_dirs = [ ".." ]
   defines = [
     # The only effect of DART_SHARED_LIB is to export the Dart API.
     "DART_SHARED_LIB",
   ]
+  if (is_linux || is_android) {
+    cflags = [ "-fPIC" ]
+  }
   if (is_win) {
     libs = [ "dart.lib" ]
     abs_root_out_dir = rebase_path(root_out_dir)
     ldflags = [ "/LIBPATH:$abs_root_out_dir" ]
   }
 }
 
 shared_library("sample_extension") {
   deps = [
     ":dart",
   ]
   sources = [
     "../../samples/sample_extension/sample_extension.cc",
     "../../samples/sample_extension/sample_extension_dllmain_win.cc",
   ]
   include_dirs = [ ".." ]
   defines = [
     # The only effect of DART_SHARED_LIB is to export the Dart API.
     "DART_SHARED_LIB",
   ]
+  if (is_linux || is_android) {
+    cflags = [ "-fPIC" ]
+  }
   if (is_win) {
     libs = [ "dart.lib" ]
     abs_root_out_dir = rebase_path(root_out_dir)
     ldflags = [ "/LIBPATH:$abs_root_out_dir" ]
   }
 }