appveyor.yml: Use x86 python only on x86

It causes the cpython tests to fail when using the x86_64 toolchain.

tachyon_module.c.obj : error LNK2019: unresolved external symbol __imp_PyLong_FromLong referenced in function phaserize
tachyon_module.c.obj : error LNK2019: unresolved external symbol __imp_PyArg_ParseTuple referenced in function phaserize
tachyon_module.c.obj : error LNK2019: unresolved external symbol __imp_PyModule_Create2 referenced in function PyInit_tachyon
c:\\python34/libs\python34.lib : warning LNK4272: library machine type 'X86' conflicts with target machine type 'x64'
ext/tachyon.pyd : fatal error LNK1120: 3 unresolved externals

.appveyor.yml

@@ -36,18 +36,22 @@ branches:
     - master
 
 install:
-  - ps: (new-object net.webclient).DownloadFile('https://dl.dropboxusercontent.com/u/37517477/ninja.exe', 'c:\python34\ninja.exe')
-  - cmd: copy c:\python34\python.exe c:\python34\python3.exe
+  # Use the x86 python only when building for x86 for the cpython tests.
+  # For all other archs (including, say, arm), use the x64 python.
+  - ps: (new-object net.webclient).DownloadFile('https://dl.dropboxusercontent.com/u/37517477/ninja.exe', 'C:\projects\meson\ninja.exe')
+  - cmd: if %arch%==x86 (set MESON_PYTHON_PATH=C:\python34) else (set MESON_PYTHON_PATH=C:\python34-x64)
+  - cmd: echo Using Python at %MESON_PYTHON_PATH%
+  - cmd: copy %MESON_PYTHON_PATH%\python.exe %MESON_PYTHON_PATH%\python3.exe
   - cmd: if %compiler%==msvc2010 ( call "C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\vcvarsall.bat" %arch% )
   - cmd: if %compiler%==msvc2015 ( call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" %arch% )
 
 build_script:
   - cmd: echo No build step.
-  - cmd: if %backend%==ninja ( "C:\python34\ninja.exe" --version ) else ( MSBuild /version & echo. )
+  - cmd: if %backend%==ninja ( ninja.exe --version ) else ( MSBuild /version & echo. )
 
 test_script:
   - cmd: echo Running tests for %arch% and %compiler% with the %backend% backend
-  - cmd: PATH=c:\python34;%PATH%; && python3 run_tests.py --backend=%backend%
+  - cmd: PATH=%cd%;%MESON_PYTHON_PATH%;%PATH%; && python3 run_tests.py --backend=%backend%
 
 on_finish:
   - appveyor PushArtifact meson-test-run.txt -DeploymentName "Text test logs"