Use $(bindir) for path to executable in mkmf

For the macOS -bundle_loader linker option, we need a path to the
Ruby exectuable. $(RUBY) is not necessarily a path since it could
be a command line invocation. That happens during build with
runruby.rb and can happen post installation if the user passes
the --ruby option to a extconf.rb. Use $(bindir) to locate
the executable instead.

Before installation, $(bindir) doesn't exist, so we need to be
able to override $(BUILTRUBY) in such situations so test-spec
and bundled extensions could build. Use a new mkmf global,
$builtruby, to do this; set it in fake.rb and in extmk.rb.

Our build system is quite complex...

ext/extmk.rb

@@ -417,7 +417,6 @@ elsif CONFIG['EXTSTATIC']
 else
   $ruby = '$(topdir)/ruby' + EXEEXT
 end
-$mflags << "BUILTRUBY=#$ruby"
 $ruby = [$ruby]
 $ruby << "-I'$(topdir)'"
 unless CROSS_COMPILING
@@ -428,6 +427,7 @@ end
 topruby = $ruby
 $ruby = topruby.join(' ')
 $mflags << "ruby=#$ruby"
+$builtruby = '$(topdir)/ruby' + EXEEXT # Must be an executable path
 
 MTIMES = [__FILE__, 'rbconfig.rb', srcdir+'/lib/mkmf.rb'].collect {|f| File.mtime(f)}
 

lib/mkmf.rb

@@ -2076,7 +2076,11 @@ sitearch = #{CONFIG['sitearch']}
 ruby_version = #{RbConfig::CONFIG['ruby_version']}
 ruby = #{$ruby.sub(%r[\A#{Regexp.quote(RbConfig::CONFIG['bindir'])}(?=/|\z)]) {'$(bindir)'}}
 RUBY = $(ruby#{sep})
-BUILTRUBY = $(RUBY)
+BUILTRUBY = #{if defined?($builtruby) && $builtruby
+    $builtruby
+  else
+    File.join('$(bindir)', CONFIG["RUBY_INSTALL_NAME"] + CONFIG['EXEEXT'])
+  end}
 ruby_headers = #{headers.join(' ')}
 
 RM = #{config_string('RM', &possible_command) || '$(RUBY) -run -e rm -- -f'}

tool/fake.rb

@@ -45,6 +45,7 @@ prehook = proc do |extmk|
   $extout_prefix = '$(extout)$(target_prefix)/'
   config = RbConfig::CONFIG
   mkconfig = RbConfig::MAKEFILE_CONFIG
+  $builtruby ||= File.join(builddir, config['RUBY_INSTALL_NAME'] + config['EXEEXT'])
   RbConfig.fire_update!("builddir", builddir)
   RbConfig.fire_update!("buildlibdir", builddir)
   RbConfig.fire_update!("libdir", builddir)