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 flavorjones#127

lib/mini_portile2/mini_portile_cmake.rb

@@ -11,9 +11,9 @@ 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 +48,16 @@ def make_cmd
   def cmake_cmd
     (ENV["CMAKE"] || @cmake_command || "cmake").dup
   end
+
+  private
+
+  def generator_available?(generator_type)
+    action = 'cmake_help'
+    execute(action, [cmake_cmd, '--help'])
+    filename = log_file(action)
+
+    raise 'Unable to determine whether CMake supports #{generator_type} Makefile generator' unless File.exist?(filename)
+
+    File.read(filename).include?("#{generator_type} Makefiles")
+  end
 end

test/test_cmake.rb

@@ -77,6 +77,48 @@ def test_make_command_configuration
     end
   end
 
+  def test_configure_defaults_with_unix_makefiles
+    script = write_cmake_help("Unix")
+
+    MiniPortile.stub(:mingw?, true) do
+      with_env("CMAKE"=>script.path) do
+        test_recipe = cmake_test_recipe
+
+        assert_equal([], test_recipe.configure_defaults)
+      end
+    end
+  ensure
+    script&.unlink
+  end
+
+  def test_configure_defaults_with_msys_makefiles
+    script = write_cmake_help("MSYS")
+
+    MiniPortile.stub(:mingw?, true) do
+      with_env("CMAKE"=>script.path) do
+        test_recipe = cmake_test_recipe
+
+        assert_equal(['-G', 'MSYS Makefiles'], test_recipe.configure_defaults)
+      end
+    end
+  ensure
+    script&.unlink
+  end
+
+  def test_configure_defaults_with_nmake_makefiles
+    script = write_cmake_help("NMake")
+
+    MiniPortile.stub(:mswin?, true) do
+      with_env("CMAKE"=>script.path) do
+        test_recipe = cmake_test_recipe
+
+        assert_equal(['-G', 'NMake Makefiles'], test_recipe.configure_defaults)
+      end
+    end
+  ensure
+    script&.unlink
+  end
+
   def test_cmake_command_configuration
     without_env("CMAKE") do
       assert_equal("cmake", MiniPortileCMake.new("test", "1.0.0").cmake_cmd)
@@ -87,4 +129,26 @@ def test_cmake_command_configuration
       assert_equal("asdf", MiniPortileCMake.new("test", "1.0.0", cmake_command: "xyzzy").cmake_cmd)
     end
   end
+
+  private
+
+  def cmake_test_recipe
+    MiniPortileCMake.new("test-cmake", "1.0") do |recipe|
+      recipe.file = File.expand_path("../../tmp/test-cmake-1.0.tar.gz", __FILE__)
+      recipe.cook
+    end
+  end
+
+  def write_cmake_help(generator_type)
+    cmake_script = Tempfile.new('cmake_help')
+    cmake_script.puts <<~SCRIPT
+      #!/bin/sh
+      echo "The following generators are available on this platform (* marks default):"
+      echo "* #{generator_type} Makefiles               = Generates standard #{generator_type.upcase} makefiles."
+    SCRIPT
+    cmake_script.close
+
+    File.chmod(0755, cmake_script)
+    cmake_script
+  end
 end