Report build configuration in test/bench output #796

1. 
   sipa commented at 10:21 pm on August 10, 2020: contributor

   Suggested here: #793 (comment)

   This should include explicitly selected configuration options (the ones passed through through libsecp256k1-config.h), but also C-detected configuration like added in #787 (endianness) and #793 (__int128).

2. 
   elichai commented at 9:32 am on August 11, 2020: contributor

   Note, if we also want to print compiler version etc there are some useful macros here: https://www.boost.org/doc/libs/1_63_0/boost/config/ (specifically BOOST_COMPILER and BOOST_PLATFORM)
3. 
   sipa commented at 3:00 pm on August 11, 2020: contributor

   @elichai Boost is C++. libsecp256k1 is C.
4. 
   elichai commented at 3:10 pm on August 11, 2020: contributor

   I didn’t mean that we should use boost, But we could look at the macros as references

Contributors
sipa elichai