Set PLATFORM to Web by default when configuring web builds with emcmake (#4748)

CMakeOptions.txt

@@ -2,6 +2,10 @@
 include(CMakeDependentOption)
 include(EnumOption)
 
+if(EMSCRIPTEN)
+    # When configuring web builds with "emcmake cmake -B build -S .", set PLATFORM to Web by default
+    SET(PLATFORM Web CACHE STRING "Platform to build for.")
+endif()
 enum_option(PLATFORM "Desktop;Web;Android;Raspberry Pi;DRM;SDL" "Platform to build for.")
 
 enum_option(OPENGL_VERSION "OFF;4.3;3.3;2.1;1.1;ES 2.0;ES 3.0" "Force a specific OpenGL Version?")