Use #pragma push_macro to properly restore the _DEBUG macro

[ikolev21]

Issue description

The common.h header uses #undef _DEBUG before including the Python headers (I don't know why, but I guess it's important).
When it later restores the macro it uses #define _DEBUG, which may be different from the original definition of the macro that happens to be 1 for example.
As a result subsequent code that expects _DEBUG to have a value will not compile.
I ran into this when including tbb.h (Threading Building Blocks) after pybind11.h, the TBB headers do expect _DEBUG to have a value.
This can be fixed by using #pragma push_macro("_DEBUG") before the #undef and #pragma pop_macro("_DEBUG") in place of the #define.
The push_macro pragma is a VC++ feature, not sure if other compilers have it, but this is done just for _MSC_VER anyway.

Reproducible example code

#include "pybind11/pybind11.h"

#if _DEBUG
#pragma message( "debug" )
#endif

[mp-t]

/// Include Python header, disable linking to pythonX_d.lib on Windows in debug mode

Why is this even happening? Why is there no comment explaining this? Why is this not configurable?

[YannickJadoul]

Related to https://bugs.python.org/issue22411, so maybe this is not needed anymore in 3.5+?
The build system and CI being refactored, so let's revisit this after that, so we have a better view on debug builds for Windows, etc.

[mbeutel]

The #undef _DEBUG currently breaks linking in debug mode when using Pybind11 with Vcpkg, so I'd appreciate if this was removed.

Edit: I need to retract that. This seems to be a larger problem, and Pybind11 is probably not the culprit here.