Define C11 macros such as FLT_DECIMAL_DIG for C++17

Define C11 macros such as FLT_DECIMAL_DIG for C++17

Backport from mainline
2019-06-24  Jonathan Wakely  <jwakely@redhat.com>

	* ginclude/float.h (FLT_DECIMAL_DIG, DBL_DECIMAL_DIG, LDBL_DECIMAL_DIG)
	(FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, LDBL_HAS_SUBNORM, FLT_TRUE_MIN)
	(DBL_TRUE_MIN, LDBL_TRUE_MIN): Also define for C++17.

Backport from mainline
2019-06-24  Jonathan Wakely  <jwakely@redhat.com>

	* testsuite/18_support/headers/cfloat/values_c++17.cc: New test.

From-SVN: r273949