Disable --as-needed for libpthread to work around linker bug.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 4b624cfe3d107f35edc763d7bb22e54819e70f3b..71a6b28e9f51e677651b6e5b273238a7774f219d 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1280,6 +1280,14 @@ if (is_win) {
       # Functions interposed by the sanitizers can make ld think
       # that some libraries aren't needed when they actually are,
       # http://crbug.com/234010. As workaround, disable --as-needed.
+      if (!is_nacl && !is_android) {

[Nico, 2016/06/28 21:56:23]

what about chromeos? should this only happen if is_clang, and/or if lto is on?

[pcc1, 2016/06/28 22:19:34]

This should probably be testing for whether the standard library is glibc, but
I'm not sure that we have a good way to test that.

It's only by chance that we saw this issue with Clang+LTO; the same issue could
happen with GCC depending on how code is inlined, or with any non-PIC object
file (although I don't think our build system can build any of those).

Regarding ChromeOS, according to [1] it uses glibc so I don't think we need to
do anything about it.

[1]
http://unix.stackexchange.com/questions/269487/is-chromium-os-a-gnu-linux-distro

+        # TODO(pcc): Fix linker bug which requires us to link pthread
+        # unconditionally here (crbug.com/623236).
+        common_optimize_on_ldflags += [
+          "-Wl,--no-as-needed",
+          "-lpthread",
+        ]
+      }
       common_optimize_on_ldflags += [ "-Wl,--as-needed" ]
     }
   }