SCons: Add 'split_libmodules' option to workaround linker issue

The new 'split_libmodules=yes' option is useful to work around linker
command line size limitations when linking a huge number of objects.
We're currently over 64k chars when linking libmodules.a on Windows
with MinGW, which triggers issues as seen in #30892.

Even on Linux, we can also reach linker command line size limitations
by adding more custom modules.

We force this option to True for MinGW on Windows, which fixes #30892.

Additional changes to lib splitting:

- Fix linking of the split module libs with interdependent symbols,
  hacking our way into LINKCOM and SHLINKCOM to set the `--start-group`
  and `--end-group` flags.
- Fix Python 3 compatibility in `methods.split_lib()`.
- Drop seemingly obsolete condition for 'msys' on 'posix'.
- Drop the unnecessary 'split_drivers' as the drivers lib is no longer
  too big since we moved all thirdparty builds to modules.

Co-authored-by: Hein-Pieter van Braam-Stewart <hp@tmm.cx>

platform/windows/detect.py

@@ -151,14 +151,14 @@ def setup_msvc_auto(env):
         env['bits'] = '64'
     else:
         env['bits'] = '32'
-    print(" Found MSVC version %s, arch %s, bits=%s" % (env['MSVC_VERSION'], env['TARGET_ARCH'], env['bits']))
+    print("Found MSVC version %s, arch %s, bits=%s" % (env['MSVC_VERSION'], env['TARGET_ARCH'], env['bits']))
     if env['TARGET_ARCH'] in ('amd64', 'x86_64'):
         env["x86_libtheora_opt_vc"] = False
 
 def setup_mingw(env):
     """Set up env for use with mingw"""
     # Nothing to do here
-    print("Using Mingw")
+    print("Using MinGW")
     pass
 
 def configure_msvc(env, manual_msvc_config):
@@ -294,7 +294,10 @@ def configure_mingw(env):
     ## Compiler configuration
 
     if (os.name == "nt"):
-        env['ENV']['TMP'] = os.environ['TMP']  # way to go scons, you can be so stupid sometimes
+        # Force splitting libmodules.a in multiple chunks to work around
+        # issues reaching the linker command line size limit, which also
+        # seem to induce huge slowdown for 'ar' (GH-30892).
+        env['split_libmodules'] = True
     else:
         env["PROGSUFFIX"] = env["PROGSUFFIX"] + ".exe"  # for linux cross-compilation