third_party/libdrm: Add -D_GNU_SOURCE_

Upstream configure.ac invokes AC_USE_SYSTEM_EXTENSIONS, which adds
this flag on Linux. We need this for the upcoming libdrm rebase
which starts using vasnprintf, which is only available when this
flag is passed.

BUG=763760

Change-Id: I296446c8493f56f6bea74925c4f85a95658f9e15
Reviewed-on: https://chromium-review.googlesource.com/737472
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512679}

DEPS

@@ -406,7 +406,7 @@ deps = {
 
   # Userspace interface to kernel DRM services.
   'src/third_party/libdrm/src': {
-      'url': Var('chromium_git') + '/chromiumos/third_party/libdrm.git' + '@' + '0ce18bedd3e62d4784fa755403801934ba171084',
+      'url': Var('chromium_git') + '/chromiumos/third_party/libdrm.git' + '@' + '16ffb1e6fce0fbd57f7a1e76021c575a40f6dc7a',
       'condition': 'checkout_linux',
   },
 

third_party/libdrm/BUILD.gn

@@ -15,6 +15,8 @@ declare_args() {
 
 if (!use_system_libdrm) {
   config("libdrm_config") {
+    # Define _GNU_SOURCE for vasprintf use in xf86drm.c
+    defines = [ "_GNU_SOURCE" ]
     include_dirs = [
       "src",
       "src/include",

third_party/libdrm/README.chromium

 Name: libdrm
 Short Name: libdrm
 URL: https://chromium.googlesource.com/chromiumos/third_party/libdrm
-Version: 2.4.70
+Version: 2.4.85
 License: MIT, GPL
 License File: NOT_SHIPPED
 Security Critical: yes