linux: Build third-party code with -Wunused-result.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index cbea95780ec9259301466486890457d51b26eb90..3875893c8ed8b057410431c6b6f092c221626d53 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -3163,15 +3163,6 @@
               '-Werror',
             ],
           }],
-          [ 'os_posix==1 and os_bsd!=1 and OS!="mac" and OS!="android"', {
-            'cflags': [
-              # Don't warn about ignoring the return value from e.g. close().
-              # This is off by default in some gccs but on by default in others.
-              # BSD systems do not support this option, since they are usually
-              # using gcc 4.2.1, which does not have this flag yet.
-              '-Wno-unused-result',
-            ],
-          }],
           [ 'OS=="win"', {
             'defines': [
               '_CRT_SECURE_NO_DEPRECATE',