appveyor: Pass the generator platform explicitly

In a future commit we want to add support for MSVC 2019. For that
version cmake requires us to pass in the architecture not part of the
generator but explicitly.

So let's pass that in always like that. This also removes the doubling
of the platform/architecture.

appveyor.yml

@@ -66,21 +66,21 @@ environment:
       platform: x64
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
-      GENERATOR: Visual Studio 14 2015 Win64
+      GENERATOR: Visual Studio 14 2015
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x64
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
-      GENERATOR: Visual Studio 15 2017 Win64
+      GENERATOR: Visual Studio 15 2017
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x64
       CXX_FLAGS: "/permissive- /std:c++latest /utf-8 /F4000000"
       LINKER_FLAGS: "/STACK:4000000"
-      GENERATOR: Visual Studio 15 2017 Win64
+      GENERATOR: Visual Studio 15 2017
 
 init:
   - cmake --version
@@ -92,12 +92,13 @@ install:
   - if "%COMPILER%"=="mingw"  set PATH=C:\projects\deps\ninja;%PATH%
   - if "%COMPILER%"=="mingw"  set PATH=C:\mingw-w64\x86_64-7.3.0-posix-seh-rt_v5-rev0\mingw64\bin;%PATH%
   - if "%COMPILER%"=="mingw"  g++ --version
+  - if "%platform%"=="x86"    set GENERATOR_PLATFORM=Win32
 
 before_build:
   # for with_win_header build, inject the inclusion of Windows.h to the single-header library
   - ps: if ($env:name -Eq "with_win_header") { $header_path = "single_include\nlohmann\json.hpp" }
   - ps: if ($env:name -Eq "with_win_header") { "#include <Windows.h>`n" + (Get-Content $header_path | Out-String) | Set-Content $header_path }
-  - cmake . -G "%GENERATOR%" -DCMAKE_BUILD_TYPE="%configuration%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_EXE_LINKER_FLAGS="%LINKER_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin"
+  - if "%GENERATOR%"=="Ninja" (cmake . -G "%GENERATOR%" -DCMAKE_BUILD_TYPE="%configuration%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_EXE_LINKER_FLAGS="%LINKER_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin") else (cmake . -G "%GENERATOR%" -A "%GENERATOR_PLATFORM%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_EXE_LINKER_FLAGS="%LINKER_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin")
 
 build_script:
   - cmake --build . --config "%configuration%"