fix build failure with recent glibc

RT #133184

pp_crypt() directly manipulates a field inside 'struct crypt_data' to work
around a bug in an ancient glibc version from circa 2002. New glibc
releases don't have this field so perl fails to compile. Make the hack
conditional on glibc version.

Stolen from a patch to the Fedora 28 distribution.

pp.c

@@ -3653,8 +3653,12 @@ PP(pp_crypt)
 #if defined(__GLIBC__) || defined(__EMX__)
 	if (PL_reentrant_buffer->_crypt_struct_buffer) {
 	    PL_reentrant_buffer->_crypt_struct_buffer->initialized = 0;
-	    /* work around glibc-2.2.5 bug */
+#if (defined(__GLIBC__) && __GLIBC__ == 2) && \
+    (defined(__GLIBC_MINOR__) && __GLIBC_MINOR__ >= 2 && __GLIBC_MINOR__ < 4)
+	    /* work around glibc-2.2.5 bug, has been fixed at some
+	     * time in glibc-2.3.X */
 	    PL_reentrant_buffer->_crypt_struct_buffer->current_saltbits = 0;
+#endif
 	}
 #endif
     }