Set -Wmissing-prototypes for clang builds (#24757)

* Set -Wmissing-prototypes for clang builds.

GCC defines -Wmissing-declarations as specialization of -Wmissing-prototypes for C++ (See https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmissing-declarations and https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmissing-prototypes) while Clang uses -Wmissing-prototypes for both languages and -Wmissing-declarations is a different diagnostic (See https://clang.llvm.org/docs/DiagnosticsReference.html#wmissing-prototypes and
https://clang.llvm.org/docs/DiagnosticsReference.html#wmissing-declarations).
Use MATCHES against CMAKE_CXX_COMPILER_ID (See https://stackoverflow.com/a/10055571, https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_ID.html) as conditional to handle both AppleClang and Clang.
Clang enables -Wmissing-declarations by default so no point in setting it only if !Clang.

See also:
https://github.com/llvm/llvm-project/issues/16660
https://reviews.llvm.org/D119361

* Make HasMatchingLanguage in Platform.macOS.mm static.

Uncovered by setting -Wmissing-prototypes flag for Clang builds in 306c277c.

* Ignore -Wmissing-prototypes for extern GetVersion.

Result of setting -Wmissing-prototypes flag for Clang builds in 306c277c.

See also: https://github.com/llvm/llvm-project/issues/94138

CMakeLists.txt

@@ -358,6 +358,11 @@ else ()
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=objc-method-access")
     endif()
 
+    # Clang -Wmissing-declarations differs from GCC, set -Wmissing-prototypes for parity
+    if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wmissing-prototypes")
+    endif()
+
     # On mingw all code is already PIC, this will avoid compiler error on redefining this option
     if (NOT MINGW)
         set(CMAKE_POSITION_INDEPENDENT_CODE ON)

src/openrct2/Version.cpp

@@ -56,13 +56,19 @@ const char gVersionInfoFull[] = OPENRCT2_NAME ", "
     ;
 
 #ifdef __EMSCRIPTEN__
-// This must be wrapped in extern "C", according to the emscripten docs, "to prevent C++ name mangling"
+    // This must be wrapped in extern "C", according to the emscripten docs, "to prevent C++ name mangling"
+    // Ignore -Wmissing-prototypes here, see https://github.com/llvm/llvm-project/issues/94138
+    #pragma clang diagnostic push
+    #pragma clang diagnostic ignored "-Wmissing-prototypes"
+
 extern "C" {
 const char* GetVersion()
 {
     return gVersionInfoFull;
 }
 }
+
+    #pragma clang diagnostic pop
 #endif
 
 NewVersionInfo GetLatestVersion()

src/openrct2/platform/Platform.macOS.mm

@@ -155,7 +155,7 @@ namespace OpenRCT2::Platform
         return false;
     }
 
-    bool HasMatchingLanguage(NSString* preferredLocale, uint16_t* languageIdentifier)
+    static bool HasMatchingLanguage(NSString* preferredLocale, uint16_t* languageIdentifier)
     {
         @autoreleasepool
         {