Modify configure to determine return value of strerror_r.

On glibc and Android's bionic, strerror_r returns char* when
_GNU_SOURCE is defined.

Add a configure check for this rather than assume glibc is the
only libc that behaves this way.

src/malloc_io.c

@@ -111,7 +111,7 @@ buferror(int err, char *buf, size_t buflen) {
 	FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM, NULL, err, 0,
 	    (LPSTR)buf, (DWORD)buflen, NULL);
 	return 0;
-#elif defined(__GLIBC__) && defined(_GNU_SOURCE)
+#elif defined(JEMALLOC_STRERROR_R_RETURNS_CHAR_WITH_GNU_SOURCE) && defined(_GNU_SOURCE)
 	char *b = strerror_r(err, buf, buflen);
 	if (b != buf) {
 		strncpy(buf, b, buflen);