cmake: Cleanup (#2535)

- Don't call enable_language before project.
See CMake docs for CMP0165.
- Remove boost-related cache variables.
It's too late to set them because Boost::headers has already been found.
- Don't build SDL3-test library.
Not used by shadps4.
- Remove /Zc:preprocessor from toml11's INTERFACE_COMPILE_OPTIONS.
Clang-cl does not support this flag and emits a lot of warnings.
- Remove dummy.cpp and make gcn a proper INTERFACE target.

externals/CMakeLists.txt

@@ -18,8 +18,6 @@ endif()
 
 # Boost
 if (NOT TARGET Boost::headers)
-    set(BOOST_ROOT "${CMAKE_SOURCE_DIR}/externals/ext-boost" CACHE STRING "")
-    set(Boost_NO_SYSTEM_PATHS ON CACHE BOOL "")
     add_subdirectory(ext-boost)
 endif()
 
@@ -77,6 +75,7 @@ endif()
 
 # SDL3
 if (NOT TARGET SDL3::SDL3)
+    set(SDL_TEST_LIBRARY OFF)
     set(SDL_PIPEWIRE OFF)
     add_subdirectory(sdl3)
 endif()
@@ -131,6 +130,14 @@ endif()
 # Toml11
 if (NOT TARGET toml11::toml11)
     add_subdirectory(toml11)
+
+    if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+      if (CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC")
+        get_target_property(_toml11_compile_options toml11 INTERFACE_COMPILE_OPTIONS)
+        list(REMOVE_ITEM _toml11_compile_options "/Zc:preprocessor")
+        set_target_properties(toml11 PROPERTIES INTERFACE_COMPILE_OPTIONS ${_toml11_compile_options})
+      endif()
+    endif()
 endif()
 
 # xxHash