build: remove _GNU_SOURCE macro definition

Compile libuv without -D_GNU_SOURCE, remove mention from the README.

The only place where it's still used is in the test suite and only
because test/test-fs.c uses struct stat directly.

README.md

@@ -94,10 +94,6 @@ To build for android:
 Note for UNIX users: compile your project with `-D_LARGEFILE_SOURCE` and
 `-D_FILE_OFFSET_BITS=64`. GYP builds take care of that automatically.
 
-Note for Linux users: compile your project with `-D_GNU_SOURCE` when you
-include `uv.h`. GYP builds take care of that automatically. If you use
-autotools, add a `AC_GNU_SOURCE` declaration to your `configure.ac`.
-
 ## Supported Platforms
 
 Microsoft Windows operating systems since Windows XP SP2. It can be built

uv.gyp

@@ -15,7 +15,6 @@
         'defines': [
           '_LARGEFILE_SOURCE',
           '_FILE_OFFSET_BITS=64',
-          '_GNU_SOURCE',
         ],
         'conditions': [
           ['OS=="solaris"', {