Avoid redefinition of BOOST_JSON_SOURCE macro

[mloskot]

../../boost/json/src.hpp:20: warning: "BOOST_JSON_SOURCE" redefined
   20 | #define BOOST_JSON_SOURCE
      |

due to

https://github.com/vinniefalco/json/blob/37d0a09242bf0d116c0cd69415034ff93fd9cc02/build/Jamfile#L14

Similarly, may users do

#define BOOST_JSON_SOURCE
#include <boost/json/src.hpp>

?

[codecov]

Codecov Report

Merging #50 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #50   +/-   ##
========================================
  Coverage    99.09%   99.09%           
========================================
  Files           57       57           
  Lines         4854     4854           
========================================
  Hits          4810     4810           
  Misses          44       44

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[vinniefalco]

Thanks!

[vinniefalco]

Thanks!