Implement "P0631R4 Math Constants" for C++20

Implement "P0631R4 Math Constants" for C++20

The values of the constants are taken from Glibc where the equivalent
constant exists, or by rounding the actual constant to the same number
of digits as the Glibc constants have.

	P0631R4 Math Constants
	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/precompiled/stdc++.h: Include new header.
	* include/std/numbers: New header.
	* include/std/version (__cpp_lib_math_constants): Define.
	* testsuite/26_numerics/numbers/1.cc: New test.
	* testsuite/26_numerics/numbers/2.cc: New test.
	* testsuite/26_numerics/numbers/3.cc: New test.
	* testsuite/26_numerics/numbers/nonfloat_neg.cc: New test.

From-SVN: r273940