Add methods for asking about compiler features

lib/mruby/build/command.rb

 require 'forwardable'
+require 'tempfile'
 
 module MRuby
   class Command
@@ -126,6 +127,31 @@ module MRuby
       end
     end
 
+    def compiles?(source_text)
+      infile = Tempfile.new ['', '.c']
+      infile.write source_text
+      infile.close
+      cwd = Dir.pwd
+      is_success = false
+      # Change to a tmp dir when compiling so we don't litter compiler artifacts
+      Dir.mktmpdir do |tmpdir|
+        Dir.chdir tmpdir
+        sh(command, infile.path, verbose: false) { |retval, _| is_success = retval }
+      end
+      infile.delete
+      Dir.chdir cwd
+
+      return is_success
+    end
+
+    def has_header?(header_name)
+      compiles? test_code_template header: header_name
+    end
+
+    def has_function?(function_name, with_header: nil)
+      compiles? test_code_template function: function_name, header: with_header
+    end
+
     private
 
     #
@@ -165,6 +191,46 @@ module MRuby
       end.flatten.uniq
       deps << MRUBY_CONFIG
     end
+
+    def test_code_template(function: nil, header: nil)
+      preamble = ''
+      body = ''
+
+      if header
+        preamble += <<-TEMPLATE
+#if defined __has_include
+#if !__has_include("#{header}")
+#error "Header #{header} not found"
+#endif
+#endif
+#include <#{header}>
+        TEMPLATE
+      end
+
+      if function
+        preamble += <<-TEMPLATE
+#if defined __stub_#{function} || defined __stub___#{function}
+#{function} unavailable
+#endif
+        TEMPLATE
+
+        # This is how autoconf works when using a function prototype.
+        body += <<-TEMPLATE
+  void *a = (void*) &#{function};
+  long long b = (long long) a;
+  return (int) b;
+        TEMPLATE
+      else
+        body = 'return 0;'
+      end
+
+      <<-TEMPLATE
+#{preamble}
+int main(void) {{
+#{body}
+}}
+      TEMPLATE
+    end
   end
 
   class Command::Linker < Command