NAMESPACE_BEGIN macro redefinition

[Trigve]

Issue description

The NAMESPACE_BEGIN is very generic name and therefor there could be the name clash (for instance CryptoPP does use the NAMESPACE_BEGIN also).

It could be fixed renaming the NAMESPACE_BEGIN -> PYBIND11_NAMESPACE_BEGIN. I think this is the only macro that hasn't the PYBIND11 prefix.

[noeffn]

Encountered this problem had to pretty much #undef NAMESPACE_BEGIN #undef NAMSPACE_END
before including pybind11

[wjakob]

This change would be fine with me. Do you want to make a PR? FWIW, CryptoPP should probably also change its macro :-P

[YannickJadoul]

Funnily enough, pybind11 behaves quite polite here, checking if no one else already defined it:

#if !defined(NAMESPACE_BEGIN)
#  define NAMESPACE_BEGIN(name) namespace name {
#endif
#if !defined(NAMESPACE_END)
#  define NAMESPACE_END(name) }
#endif

[YannickJadoul]

But, see #2283.