Update icu4c/source/i18n/messageformat2_function_registry.cpp

Co-authored-by: Markus Scherer <[email protected]>

icu4c/source/i18n/messageformat2_function_registry.cpp

@@ -19,8 +19,12 @@
 #include "number_types.h"
 #include "uvector.h" // U_ASSERT
 
+// The C99 standard suggested that C++ implementations not define PRId64 etc. constants
+// unless this macro is defined.
+// See the Notes at https://en.cppreference.com/w/cpp/types/integer .
+// Similar to defining __STDC_LIMIT_MACROS in unicode/ptypes.h .
 #ifndef __STDC_FORMAT_MACROS
-#define __STDC_FORMAT_MACROS
+#   define __STDC_FORMAT_MACROS
 #endif
 #include <inttypes.h>
 #include <math.h>