Implemented BOOST_PFR_ENABLED macro

[denzor200]

New macro tested on the Godbolt in so many variants:
case 1 (before) : https://godbolt.org/z/hjhKf3h5n
case 1 (after) : https://godbolt.org/z/vfeKfcMh7
case 2 (before) : https://godbolt.org/z/bvfocY1v1
case 2 (after) : https://godbolt.org/z/bnYboGYa5
case 3 (before) : https://godbolt.org/z/es7cEoo75
case 3 (after) : https://godbolt.org/z/sMdr1vGPW

[coveralls]

Pull Request Test Coverage Report for Build 954428715

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 931559387:	0.0%
Covered Lines:	332
Relevant Lines:	332

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3725964878

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 3725610787:	0.0%
Covered Lines:	337
Relevant Lines:	337

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💛 - Coveralls

[denzor200]

I think this is a bad idea.. But we will not close it yet

[denzor200]

I think this is a bad idea.. But we will not close it yet

This idea may become good if we provide the way to override compiler's decision manually. Like this:

#define BOOST_NO_PFR 1    //< force set to 'disallowed'
#include <boost/pfr.hpp>  //< compile time error, PFR not supported by user's decision

#define BOOST_NO_PFR 0    //< force set to 'allowed'
#include <boost/pfr.hpp>  //< good when your compiler really supports pfr

// #define BOOST_NO_PFR 0  //< let compiler set PFR's status
#include <boost/pfr.hpp>   //< good when your compiler support pfr

I'll continue working on it.