Tested with

  make check RUNTESTFLAGS="conformance.exp=*numeric*synopsis* --target_board=unix/-std=$std"

for std in {c++98, c++11, c++17, c++2a}.

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/headers/numeric/synopsis.cc: Add signatures for
	functions introduced in C++11, C++17 and C++2a.  Add 'constexpr' to
	existing signatures for C++2a.