cmake: only use MSYS/NMake generators when available

When cross-compiling Windows targets with `cmake` in a Linux
environment, the MSYS generator may not be available. Supplying `-G
MSYS` will cause the build to fail.

`cmake --help` will output the available generators. Before including
the `-G` option, check that the generator is available.

Closes #127

lib/mini_portile2/mini_portile_cmake.rb

@@ -1,4 +1,5 @@
 require 'mini_portile2/mini_portile'
+require 'open3'
 
 class MiniPortileCMake < MiniPortile
   def configure_prefix
@@ -11,9 +12,9 @@ def initialize(name, version, **kwargs)
   end
 
   def configure_defaults
-    if MiniPortile.mswin?
+    if MiniPortile.mswin? && generator_available?('NMake')
       ['-G', 'NMake Makefiles']
-    elsif MiniPortile.mingw?
+    elsif MiniPortile.mingw? && generator_available?('MSYS')
       ['-G', 'MSYS Makefiles']
     else
       []
@@ -48,4 +49,14 @@ def make_cmd
   def cmake_cmd
     (ENV["CMAKE"] || @cmake_command || "cmake").dup
   end
+
+  private
+
+  def generator_available?(generator_type)
+    stdout_str, status = Open3.capture2("#{cmake_cmd} --help")
+
+    raise 'Unable to determine whether CMake supports #{generator_type} Makefile generator' unless status.success?
+
+    stdout_str.include?("#{generator_type} Makefiles")
+  end
 end

test/test_cmake.rb

@@ -59,7 +59,7 @@ def test_install
   end
 end
 
-class TestCMakeConfig < TestCase
+class TestCMakeConfig < TestCMake
   def test_make_command_configuration
     MiniPortile.stub(:mswin?, false) do
       without_env("MAKE") do
@@ -77,6 +77,30 @@ def test_make_command_configuration
     end
   end
 
+  def test_configure_defaults_with_unix_makefiles
+    Open3.stub(:capture2, cmake_help_mock('Unix')) do
+      MiniPortile.stub(:mingw?, true) do
+        assert_equal([], @recipe.configure_defaults)
+      end
+    end
+  end
+
+  def test_configure_defaults_with_msys_makefiles
+    Open3.stub(:capture2, cmake_help_mock('MSYS')) do
+      MiniPortile.stub(:mingw?, true) do
+        assert_equal(['-G', 'MSYS Makefiles'], @recipe.configure_defaults)
+      end
+    end
+  end
+
+  def test_configure_defaults_with_nmake_makefiles
+    Open3.stub(:capture2, cmake_help_mock('NMake')) do
+      MiniPortile.stub(:mswin?, true) do
+        assert_equal(['-G', 'NMake Makefiles'], @recipe.configure_defaults)
+      end
+    end
+  end
+
   def test_cmake_command_configuration
     without_env("CMAKE") do
       assert_equal("cmake", MiniPortileCMake.new("test", "1.0.0").cmake_cmd)
@@ -87,4 +111,20 @@ def test_cmake_command_configuration
       assert_equal("asdf", MiniPortileCMake.new("test", "1.0.0", cmake_command: "xyzzy").cmake_cmd)
     end
   end
+
+  private
+
+  def cmake_help_mock(generator_type)
+    open3_mock = MiniTest::Mock.new
+    cmake_script = <<~SCRIPT
+    echo "The following generators are available on this platform (* marks default):"
+    echo "* #{generator_type} Makefiles               = Generates standard #{generator_type.upcase} makefiles."
+  SCRIPT
+
+    exit_status = MiniTest::Mock.new
+    exit_status.expect(:success?, true)
+    expected_output = [cmake_script, exit_status]
+    open3_mock.expect(:call, expected_output, ['cmake --help'])
+    open3_mock
+  end
 end