Work around lack of embed in clang on OpenBSD and Amazon Linux 2.

Package.swift

@@ -466,6 +466,9 @@ extension Array where Element == PackageDescription.CXXSetting {
 
       .define("SWT_NO_LEGACY_TEST_DISCOVERY", .whenEmbedded()),
       .define("SWT_NO_LIBDISPATCH", .whenEmbedded()),
+
+      // OpenBSD's version of clang doesn't support __has_embed.
+      .define("SWT_TESTING_LIBRARY_VERSION", to: "0", .when(platforms: [.openbsd]))
     ]
 
     // Capture the testing library's commit info as C++ constants.

Sources/_TestingInternals/Versions.cpp

@@ -18,7 +18,8 @@ const char *swt_getTestingLibraryVersion(void) {
 #if defined(SWT_TESTING_LIBRARY_VERSION)
   // The current environment explicitly specifies a version string to return.
   return SWT_TESTING_LIBRARY_VERSION;
-#elif __has_embed("../../VERSION.txt")
+#elif __clang_major__ >= 17 && defined(__has_embed)
+#if __has_embed("../../VERSION.txt")
   static constinit auto version = [] () constexpr {
     // Read the version from version.txt at the root of the package's repo.
     char version[] = {
@@ -43,6 +44,10 @@ const char *swt_getTestingLibraryVersion(void) {
 #warning SWT_TESTING_LIBRARY_VERSION not defined and VERSION.txt not found: testing library version is unavailable
   return nullptr;
 #endif
+#else
+#warning SWT_TESTING_LIBRARY_VERSION not defined and could not read from VERSION.txt at compile time: testing library version is unavailable
+  return nullptr;
+#endif
 }
 
 void swt_getTestingLibraryCommit(const char *_Nullable *_Nonnull outHash, bool *outModified) {

cmake/modules/shared/CompilerSettings.cmake

@@ -42,3 +42,8 @@ if(CMAKE_SYSTEM_NAME STREQUAL "WASI")
   add_compile_definitions("SWT_NO_DYNAMIC_LINKING")
   add_compile_definitions("SWT_NO_PIPES")
 endif()
+
+# Avoid using C23's #embed when building with CMake as OpenBSD and Amazon Linux
+# 2 are both using older clang versions that don't support it.
+file(STRINGS "../VERSION.txt" SWT_TESTING_LIBRARY_VERSION LIMIT_COUNT 1)
+add_compile_definitions("$<$<COMPILE_LANGUAGE:CXX>:SWT_TESTING_LIBRARY_VERSION=\"${SWT_TESTING_LIBRARY_VERSION}\">")