Macroizing abort is non-conformant

[StephanTLavavej]

This code is non-conformant and fails to compile with the current development version of Visual Studio:

stacktrace/example/assert_handler.cpp

Lines 27 to 34 in 211d291

	namespace std { inline void ignore_abort(){ std::exit(0); } }
	#define abort ignore_abort
	//[getting_started_assert_handlers
	// BOOST_ENABLE_ASSERT_DEBUG_HANDLER is defined for the whole project
	#include <stdexcept> // std::logic_error
	#include <iostream> // std::cerr

First, implementing ignore_abort in namespace std is forbidden by N4842 16.5.4.2.1 [namespace.std]/1: "Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std."

Second, macroizing abort is extremely forbidden by 16.5.4.3.2 [macro.names]/1: "A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header."

Third, this is failing to compile for the current development version of MSVC's STL because including <iostream> drags in <istream>, <ostream>, <ios>, internal <xlocnum>, and finally <cmath> where our implementation of lerp() contains a call to ::abort() which is broken by the macroized abort: https://github.com/microsoft/STL/blob/94c9f9965b4e39bab9fdec6f71f1fad320e96d8b/stl/inc/cmath#L1379-L1380

The assert_handler.cpp example should be changed to follow the Standard's requirements.

[apolukhin]

Fixed in 178d287, merged to master.

Sorry for the mess