build: don't link against liblog on host system

Don't link binaries that run on the host system against liblog, it
breaks cross-compiling for android.

Fixes: https://github.com/nodejs/node/issues/7731
PR-URL: https://github.com/nodejs/node/pull/7762
Reviewed-By: Anna Henningsen <anna@addaleax.net>

common.gypi

@@ -301,10 +301,14 @@
           }],
         ],
       }],
-      [ 'OS=="android"', {
+      ['OS=="android"', {
-        'defines': ['_GLIBCXX_USE_C99_MATH'],
+        'target_conditions': [
+          ['_toolset=="target"', {
+            'defines': [ '_GLIBCXX_USE_C99_MATH' ],
             'libraries': [ '-llog' ],
           }],
+        ],
+      }],
       ['OS=="mac"', {
         'defines': ['_DARWIN_USE_64_BIT_INODE=1'],
         'xcode_settings': {