Have a Perl_isfinitel() separate from USE_LONG_DOUBLE.

(Also correct uses of HAS_C99.)
long double / NV schizophrenia continues.
Use -fstack-protector-strong, if available.

In preference to just -fstack-protector. gcc 4.8.x? gcc 4.9, definitely.

(FWIW, users include at least Google Chrome, Fedora 20, Ubuntu)