Fix C++11/C++14 build with -fno-exceptions

[viordash]

Problem:

• Compilation fails with C++11/C++14 and -fno-exceptions due to missing include for std::terminate().

STR:

$ g++ --version
g++ (Ubuntu 11.4.0-1ubuntu1~22.04.2) 11.4.0

$ g++ -std=c++14 -Wall -Wextra -fno-exceptions -g -O0 -o 01-basic.exe 01-basic.cpp && ./01-basic.exe
In file included from 01-basic.cpp:1:
nonstd/span.hpp: In function ‘void nonstd::span_lite::detail::report_contract_violation(const char*)’:
nonstd/span.hpp:863:10: error: ‘terminate’ is not a member of ‘std’
  863 |     std::terminate();
      |          ^~~~~~~~~

Solution:

• Add #include when using std::terminate() in contract violation handling.

[tjenssen]

With exceptions enabled (span_CONFIG_NO_EXCEPTIONS == 0) and default contract behavior
(span_CONFIG_CONTRACT_VIOLATION_THROWS_V == 0), report_contract_violation() calls
std::terminate(), but is not included (only stdexcept).

137:
#ifndef span_CONFIG_NO_EXCEPTIONS
# if defined(_MSC_VER)
#  include <cstddef>    // for _HAS_EXCEPTIONS
# endif
# if defined(__cpp_exceptions) || defined(__EXCEPTIONS) || (_HAS_EXCEPTIONS)
#  define span_CONFIG_NO_EXCEPTIONS  0
# else
#  define span_CONFIG_NO_EXCEPTIONS  1
#  undef  span_CONFIG_CONTRACT_VIOLATION_THROWS
#  undef  span_CONFIG_CONTRACT_VIOLATION_TERMINATES
#  define span_CONFIG_CONTRACT_VIOLATION_THROWS  0
#  define span_CONFIG_CONTRACT_VIOLATION_TERMINATES  1
# endif
#endif

166:
#if    defined( span_CONFIG_CONTRACT_VIOLATION_THROWS )
# define        span_CONFIG_CONTRACT_VIOLATION_THROWS_V  span_CONFIG_CONTRACT_VIOLATION_THROWS
#else
# define        span_CONFIG_CONTRACT_VIOLATION_THROWS_V  0
#endif


494:
#if ! span_CONFIG( NO_EXCEPTIONS )
# include <stdexcept>
#elif ! span_CONFIG( CONTRACT_VIOLATION_THROWS_V )
# include <exception>
#endif

844:
#if span_CONFIG( CONTRACT_VIOLATION_THROWS_V )

struct contract_violation : std::logic_error
{
    explicit contract_violation( char const * const message )
        : std::logic_error( message )
    {}
};

inline void report_contract_violation( char const * msg )
{
    throw contract_violation( msg );
}

#else // span_CONFIG( CONTRACT_VIOLATION_THROWS_V )

span_noreturn inline void report_contract_violation( char const * /*msg*/ ) span_noexcept
{
    std::terminate();
}

[martinmoene]

@tjenssen Thanks for the heads-up, I'll have a look at it.