diff --git a/include/string.h b/include/string.h
index 5f14c37e4..a1ce56cbc 100644
--- a/include/string.h
+++ b/include/string.h
@@ -58,17 +58,13 @@ size_t strlen(const char* s);
 #pragma GCC system_header
 #endif
 
-#ifdef __cplusplus
-#define __CORRECT_ISO_CPP_STRING_H_PROTO
-#endif
-
 #include_next <string.h>
 
-// MSVCRT, GNU libc and its derivates already have the correct prototype in
-// <string.h> if __cplusplus is defined. This macro can be defined by users if
-// their C library provides the right signature.
-#if defined(__GLIBC__) || defined(_LIBCPP_MSVCRT) || defined(__sun__) || \
-    defined(_STRING_H_CPLUSPLUS_98_CONFORMANCE_)
+// MSVCRT, GNU libc and its derivates may already have the correct prototype in
+// <string.h>. This macro can be defined by users if their C library provides
+// the right signature.
+#if defined(__CORRECT_ISO_CPP_STRING_H_PROTO) || defined(_LIBCPP_MSVCRT) || \
+    defined(__sun__) || defined(_STRING_H_CPLUSPLUS_98_CONFORMANCE_)
 #define _LIBCPP_STRING_H_HAS_CONST_OVERLOADS
 #endif