Avoid UNICODE characters in TMP env var

Some people have Unicode characters in their TMP path.
This is not properly handled by gcc, when it creates temporary files, so that it fails to compile.

As a work-around we verify that TMP contains only ACSII characters, and if not set it to MSYS2's /tmp.
This variable es NOT restored by "ridk disable", because the cmd and powershell mechanism to set variables doesn't support Unicode either.

Fixes #320

lib/ruby_installer/build/msys2_installation.rb

@@ -149,6 +149,10 @@ def disable_dll_search_paths
       vars['ACLOCAL_PATH'] = "#{mingw_prefix}/share/aclocal:/usr/share/aclocal"
       vars['MANPATH'] = "#{mingw_prefix}/share/man"
       vars['MINGW_PACKAGE_PREFIX'] = mingw_package_prefix
+      if ENV['TMP'] !~ /\A[ -~]*\z/
+        # TMP has some Unicode characters -> use MSYS2's /tmp to work around incompat with gcc's temporary files
+        vars['TMP'] = backslachs( msys_path + "/tmp" )
+      end
 
       case mingwarch
         when 'mingw32'