omit declaration of basename wrongly interpreted as prototype in C++

the non-prototype declaration of basename in string.h is an ugly
compromise to avoid breaking 2 types of broken software:

1. programs which assume basename is declared in string.h and thus
would suffer from dangerous pointer-truncation if an implicit
declaration were used.

2. programs which include string.h with _GNU_SOURCE defined but then
declare their own prototype for basename using the incorrect GNU
signature for the function (which would clash with a correct
prototype).

however, since C++ does not have non-prototype declarations and
interprets them as prototypes for a function with no arguments, we
must omit it when compiling C++ code. thankfully, all known broken
apps that suffer from the above issues are written in C, not C++.

include/string.h

@@ -85,8 +85,10 @@ char *strcasestr(const char *, const char *);
 char *strsep(char **, const char *);
 void *memrchr(const void *, int, size_t);
 void *mempcpy(void *, const void *, size_t);
+#ifndef __cplusplus
 char *basename();
 #endif
+#endif
 
 #ifdef __cplusplus
 }