build: Fix linking demos on Visual Studio

The demos are now built as GUI programs, which will require the presence
of WinMain() on Visual Studio builds, unless we specify the entry point.

Pass the /entry:mainCRTStartup linker flag on Visual Studio builds for
the demo programs so that they can link properly.

https://bugzilla.gnome.org/show_bug.cgi?id=773299

demos/gtk-demo/meson.build

@@ -104,6 +104,7 @@ executable('gtk4-demo',
            dependencies: gtkdemo_deps,
            include_directories: confinc,
            gui_app: true,
+           link_args: extra_demo_ldflags,
            install: true)
 
 executable('gtk4-demo-application',
@@ -112,6 +113,7 @@ executable('gtk4-demo-application',
            dependencies: gtkdemo_deps,
            include_directories: confinc,
            gui_app: true,
+           link_args: extra_demo_ldflags,
            install: true)
 
 # icons

demos/icon-browser/meson.build

@@ -14,6 +14,7 @@ executable('gtk4-icon-browser',
            dependencies: libgtk_dep,
            include_directories: confinc,
            gui_app: true,
+           link_args: extra_demo_ldflags,
            install: true)
 
 install_data('gtk4-icon-browser.desktop', install_dir: gtk_applicationsdir)

demos/widget-factory/meson.build

@@ -9,6 +9,7 @@ executable('gtk4-widget-factory',
            dependencies: libgtk_dep,
            include_directories: confinc,
            gui_app: true,
+           link_args: extra_demo_ldflags,
            install: true)
 
 # desktop file

meson.build

@@ -559,9 +559,15 @@ if quartz_enabled
   backend_immodules += ['quartz']
 endif
 
+extra_demo_ldflags = []
 if win32_enabled
   pc_gdk_extra_libs += ['-lgdi32', '-limm32', '-lshell32', '-lole32']
-  if cc.get_id() != 'msvc'
+  if cc.get_id() == 'msvc'
+    # Since the demo programs are now built as pure GUI programs, we
+    # need to pass in /entry:mainCRTStartup so that they will properly
+    # link on Visual Studio builds
+    extra_demo_ldflags = ['/entry:mainCRTStartup']
+  else
     pc_gdk_extra_libs += ['-Wl,-luuid']
   endif
   pc_gdk_extra_libs += ['-lwinmm', '-ldwmapi', '-lsetupapi', '-lcfgmgr32']