project tests: Fix appveyor overwriting the platform env variable

There is no way to do this in the .appveyor.yml file since it seems that
the appveyor environment is forcibly written after each cmd command that
is run.

run_project_tests.py

@@ -512,6 +512,21 @@ if __name__ == '__main__':
     options = parser.parse_args()
     setup_commands(options.backend)
 
+    # Appveyor sets the `platform` environment variable which completely messes
+    # up building with the vs2010 and vs2015 backends.
+    #
+    # Specifically, MSBuild reads the `platform` environment variable to set
+    # the configured value for the platform (Win32/x64/arm), which breaks x86
+    # builds.
+    #
+    # Appveyor setting this also breaks our 'native build arch' detection for
+    # Windows in environment.py:detect_windows_arch() by overwriting the value
+    # of `platform` set by vcvarsall.bat.
+    #
+    # While building for x86, `platform` should be unset.
+    if 'APPVEYOR' in os.environ and os.environ['arch'] == 'x86':
+        os.environ.pop('platform')
+
     script_dir = os.path.split(__file__)[0]
     if script_dir != '':
         os.chdir(script_dir)