Update the gyp Linux build:

third_party/libxml/libxml.gyp

 # Copyright (c) 2009 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'conditions': [
       ['OS=="linux"', {'os_include': 'linux'}],
       ['OS=="mac"', {'os_include': 'mac'}],
       ['OS=="win"', {'os_include': 'win32'}],
@@ skipping 155 matching lines @@
       'type': 'executable',
       'sources': [
         'xmlcatalog.c',
       ],
       'include_dirs': [
         '<(os_include)',
       ],
       'dependencies': [
         'libxml',
       ],
+      'conditions': [
+        ['OS=="linux"', {
+          'link_settings': {
+            'libraries': [
+              '-lm',
+            ],
+          },
+        }],
+      ],
     },
     {
       'target_name': 'xmllint',
       'type': 'executable',
       'sources': [
         'xmllint.c',
       ],
       'include_dirs': [
         '<(os_include)',
       ],
       'dependencies': [
         'libxml',
       ],
+      'conditions': [
+        ['OS=="linux"', {
+          'link_settings': {
+            'libraries': [
+              '-lm',
+            ],
+          },
+        }],
+      ],
     },
   ],
 }