Add build matrix to appveyor.yml

This will break out MinGW, VS2015, and VS2017 builds into separate jobs.
While they will still probably build in serial, we'll at least get better
granularity on test results.

appveyor.yml

@@ -7,8 +7,8 @@
 #     support for.
 #   - Use CMake to ...
 #       build Cap'n Proto with MinGW.
-#       build Cap'n Proto with VS2015.
-#       build Cap'n Proto samples with VS2015.
+#       build Cap'n Proto with VS2015 and VS2017.
+#       build Cap'n Proto samples with VS2015 and VS2017.
 
 version: "{build}"
 
@@ -32,6 +32,23 @@ environment:
   MINGW_ARCHIVE: x86_64-4.8.5-release-win32-seh-rt_v4-rev0.7z
   BUILD_TYPE: debug
 
+  matrix:
+    # TODO(someday): Add MSVC x64 builds, MinGW x86 build?
+
+    - CMAKE_GENERATOR: Visual Studio 15 2017
+      BUILD_NAME: vs2017
+      EXTRA_BUILD_FLAGS: # /maxcpucount
+      # TODO(someday): Right now /maxcpucount occasionally expresses a filesystem-related race:
+      #   capnp-capnpc++ complains that it can't create test.capnp.h.
+
+    - CMAKE_GENERATOR: Visual Studio 14 2015
+      BUILD_NAME: vs2015
+      EXTRA_BUILD_FLAGS: # /maxcpucount
+
+    - CMAKE_GENERATOR: MinGW Makefiles
+      BUILD_NAME: mingw
+      EXTRA_BUILD_FLAGS: -j2
+
 install:
   - if not exist "%MINGW_ARCHIVE%" appveyor DownloadFile "%MINGW_URL%" -FileName "%MINGW_ARCHIVE%"
   - 7z x -y "%MINGW_ARCHIVE%" > nul
@@ -40,34 +57,30 @@ install:
 
 before_build:
   - set PATH=%CD%\%MINGW_DIR%\bin;%PATH%
-  - set INSTALL_PREFIX_MINGW=%CD%\capnproto-c++-mingw
-  - set INSTALL_PREFIX_VS2015=%CD%\capnproto-c++-vs2015
+  - set BUILD_DIR=build-%BUILD_NAME%
+  - set INSTALL_PREFIX=%CD%\capnproto-c++-%BUILD_NAME%
   - cmake --version
 
 build_script:
-  - echo "Building Cap'n Proto with MinGW"
+  - echo "Building Cap'n Proto with %CMAKE_GENERATOR%"
   - >-
-      cmake -Hc++ -Bbuild-mingw -G "MinGW Makefiles"
+      cmake -Hc++ -B%BUILD_DIR% -G "%CMAKE_GENERATOR%"
       -DCMAKE_BUILD_TYPE=%BUILD_TYPE%
-      -DCMAKE_INSTALL_PREFIX=%INSTALL_PREFIX_MINGW%
-  - cmake --build build-mingw --target install -- -j%NUMBER_OF_PROCESSORS%
+      -DCMAKE_INSTALL_PREFIX=%INSTALL_PREFIX%
+  - cmake --build %BUILD_DIR% --config %BUILD_TYPE% --target install -- %EXTRA_BUILD_FLAGS%
+  # MinGW wants the build type at configure-time while MSVC wants the build type at build-time. We
+  # can satisfy both by passing the build type to both cmake invocations. We have to suffer a
+  # warning, but both generators will work.
 
-  - echo "Building Cap'n Proto with Visual Studio 2015"
+  - echo "Building Cap'n Proto samples with %CMAKE_GENERATOR%"
   - >-
-      cmake -Hc++ -Bbuild-vs2015 -G "Visual Studio 14 2015" -A x64
-      -DCMAKE_INSTALL_PREFIX=%INSTALL_PREFIX_VS2015%
-  - cmake --build build-vs2015 --config %BUILD_TYPE% --target install
-  # TODO(someday): pass `-- /maxcpucount` for a parallel build. Right now it occasionally expresses
-  # a filesystem-related race: capnp-capnpc++ complains that it can't create test.capnp.h.
-
-  - echo "Building Cap'n Proto samples with Visual Studio 2015"
-  - >-
-      cmake -Hc++/samples -Bbuild-vs2015-samples -G "Visual Studio 14 2015" -A x64
-      -DCMAKE_PREFIX_PATH=%INSTALL_PREFIX_VS2015%
-  - cmake --build build-vs2015-samples --config %BUILD_TYPE%
+      cmake -Hc++/samples -B%BUILD_DIR%-samples -G "%CMAKE_GENERATOR%"
+      -DCMAKE_BUILD_TYPE=%BUILD_TYPE%
+      -DCMAKE_PREFIX_PATH=%INSTALL_PREFIX%
+  - cmake --build %BUILD_DIR%-samples --config %BUILD_TYPE%
 
 test_script:
   - timeout /t 2
   # Sleep a little to prevent interleaving test output with build output.
-  - cd build-vs2015\src
+  - cd %BUILD_DIR%\src
   - ctest -V -C %BUILD_TYPE%