Use default locale encode for writing file names in build.opt

Continue using "utf-8" to read or write "C/C++" source filess.

Tested on Windows 10 (en-US) with Arduino IDE 2.0.3 Under an
account with a diacritic charcter in the user ID path.

Needs testing on Japanese Windows

tools/mkbuildoptglobals.py

@@ -192,6 +192,8 @@
 import textwrap
 import time
 
+import locale
+
 # Need to work on signature line used for match to avoid conflicts with
 # existing embedded documentation methods.
 build_opt_signature = "/*@create-file:build.opt@"
@@ -296,14 +298,22 @@ def copy_create_build_file(source_fqfn, build_target_fqfn):
                 pass
     return True     # file changed
 
-
 def add_include_line(build_opt_fqfn, include_fqfn):
+    # Given that GCC will insert (or process as if interted) these lines into
+    # the command line. I assume that the contents of @file need to be encoded
+    # to match that of the shell running GCC runs. I am not 100% sure this API
+    # gives me that, but it appears to work.
+    #
+    # However, elsewhere when dealing with source code we continue to use 'utf-8',
+    # ref. https://gcc.gnu.org/onlinedocs/gcc-4.1.2/cpp/Character-sets.html
+    shell_encoding = locale.getdefaultlocale()[1]
     if not os.path.exists(include_fqfn):
         # If file is missing, we need an place holder
-        with open(include_fqfn, 'w', encoding="utf-8"):
+        with open(include_fqfn, 'w', encoding=shell_encoding):
             pass
-        print("add_include_line: Created " + include_fqfn)
-    with open(build_opt_fqfn, 'a', encoding="utf-8") as build_opt:
+        print_msg("add_include_line: Created " + include_fqfn)
+
+    with open(build_opt_fqfn, 'a', encoding=shell_encoding) as build_opt:
         build_opt.write('-include "' + include_fqfn.replace('\\', '\\\\') + '"\n')
 
 def extract_create_build_opt_file(globals_h_fqfn, file_name, build_opt_fqfn):
@@ -611,6 +621,9 @@ def main():
     global debug_enabled
     num_include_lines = 1
 
+    def_locale = locale.getdefaultlocale()
+    print_msg(f'default locale: {def_locale}')
+
     args = parse_args()
     debug_enabled = args.debug
     runtime_ide_path = os.path.normpath(args.runtime_ide_path)