build: use --subsystem,windows + main entry on MinGW (not -mwindows)

The previous two commits (SDL_MAIN_HANDLED in decdce0, dropping
-lSDL*main in 80f343b) stopped main being renamed to SDL_main and
removed SDL's WinMain shim. That exposed the underlying issue: the
MinGW build links with -mwindows, which selects crtexewin.o whose
startup calls WinMain -- and RetroArch has no WinMain. With the SDL
shim gone, the link now fails the other way:

  libmingw32.a(crtexewin.o): in function `main':
  undefined reference to `WinMain'

RetroArch provides main() (retroarch.c) and creates its own Win32
window; it never had a WinMain. The MSVC and griffin builds already
handle this with '-SUBSYSTEM:WINDOWS -ENTRY:mainCRTStartup': GUI
subsystem (no console window) but an entry point that calls main().

Mirror that on GNU/MinGW by replacing -mwindows with
-Wl,--subsystem,windows. This sets the PE subsystem to GUI (so no
console window appears, same as before) while keeping the default
crtexe.o startup, whose entry calls main(). The build then needs
neither a WinMain nor SDL's WinMain shim, and the entry model matches
the MSVC/griffin builds.

RetroArch links the Win32 import libraries it uses explicitly
(gdi32, ole32, winmm, comdlg32, dinput8, ...), so it does not depend
on -mwindows to pull those in.

Applies to both the C and CXX_BUILD MinGW builds (it is gated only on
Win32, not on CXX_BUILD), unifying their entry-point handling.

Completes the SDL entry-point series: decdce0 / 80f343b / this.

Makefile

@@ -108,7 +108,20 @@ ifeq ($(HAVE_DRMINGW), 1)
 endif
 
 ifneq ($(findstring Win32,$(OS)),)
-   LDFLAGS += -mwindows
+   # Use the GUI subsystem (no console window) but keep the default
+   # CRT entry that calls main(), rather than -mwindows which selects
+   # crtexewin.o and requires a WinMain. RetroArch provides main() (in
+   # retroarch.c) and its own Win32 window; it has no WinMain. This
+   # mirrors the MSVC/griffin builds, which use
+   # '-SUBSYSTEM:WINDOWS -ENTRY:mainCRTStartup' for the same effect.
+   # Doing it this way means the build needs neither a hand-written
+   # WinMain nor SDL's WinMain shim library (-lSDL*main), and it lets
+   # the CXX_BUILD=1 link resolve (-mwindows -> WinMain -> SDL_main was
+   # the chain that broke once main correctly stayed main). RetroArch
+   # links the Win32 libraries it needs explicitly (gdi32, ole32,
+   # winmm, comdlg32, ...), so it does not rely on -mwindows to pull
+   # them in.
+   LDFLAGS += -Wl,--subsystem,windows
 endif
 
 ifneq ($(CXX_BUILD), 1)