Enable Mathematical Special Functions for C++17

Enable Mathematical Special Functions for C++17

	* include/bits/c++config (_GLIBCXX_USE_STD_SPEC_FUNCS): Define for
	C++17, or for C++11/C++14 when __STDCPP_WANT_MATH_SPEC_FUNCS__ is
	true.
	* include/bits/specfun.h [!__STDCPP_WANT_MATH_SPEC_FUNCS__]: Don't
	do #error for C++17.
	* include/c_global/cmath: Check _GLIBCXX_USE_STD_SPEC_FUNCS instead
	of __STDCPP_WANT_MATH_SPEC_FUNCS__.
	* include/tr1/bessel_function.tcc: Likewise.
	* include/tr1/beta_function.tcc: Likewise.
	* include/tr1/cmath: Likewise.
	* include/tr1/ell_integral.tcc: Likewise.
	* include/tr1/exp_integral.tcc: Likewise.
	* include/tr1/gamma.tcc: Likewise.
	* include/tr1/hypergeometric.tcc: Likewise.
	* include/tr1/legendre_function.tcc: Likewise.
	* include/tr1/modified_bessel_func.tcc: Likewise.
	* include/tr1/poly_hermite.tcc: Likewise.
	* include/tr1/poly_laguerre.tcc: Likewise.
	* include/tr1/riemann_zeta.tcc: Likewise.
	* include/tr1/special_function_util.h: Likewise.
	* testsuite/26_numerics/headers/cmath/functions_std_c++17.cc: New.

From-SVN: r239081