opm-common/external/fmtlib

The include/ directory is a copy of the include directory from version 7.0.3 of
the fmtlib distribution. The fmtlib can be found at https://github.com/fmtlib/fmt

The fmtlib code embedded here should be compiled in header only mode, to ensure
that the symbol FMT_HEADER_ONLY must be defined before the the fmt/format.h
header is included:

   #define FMT_HEADER_ONLY
   #include <fmt/format.h>
   
   ....
   auto msg = fmt::format("Hello {}", "world");