Fully specify libmicrohttpd.a path in build.

gyp/microhttpd.gyp

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 # A simple gyp file to generate microhttpd for internal purposes
 # most of the work(configure and make) is performed in a python script
 {
   'targets': [
     {
       'target_name': 'microhttpd',
       'type': 'none',
       'variables': {
         'base_dir%': '../third_party/libmicrohttpd',
         'out_dir%': '<(INTERMEDIATE_DIR)/build',
         'src_dir%': '../third_party/externals/microhttpd',
       },
       'direct_dependent_settings': {
         'include_dirs': [
           '<(src_dir)/src/include',
         ],
         # Link the built library to dependents.
         'libraries': [
-          'libmicrohttpd.a',
+          '<(PRODUCT_DIR)/libmicrohttpd.a',
         ],
       },
       'actions': [
         {
           'action_name': 'configure_and_build',
           'inputs': [
             '<(base_dir)/build.py',
             '<(src_dir)/.git/HEAD', # This does not support local changes, but does support DEPS.
           ],
           'outputs': [ '<(PRODUCT_DIR)/libmicrohttpd.a' ],
           'action': [
             'python',
             '<(base_dir)/build.py', 
             '--src', '<(src_dir)',
             '--out', '<(out_dir)',
             '--dst', '<(PRODUCT_DIR)',
           ],
         },
       ],
     },
   ],
 }