Report build configuration in test/bench output

[sipa]

Suggested here: https://github.com/bitcoin-core/secp256k1/pull/793#issuecomment-671322287

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).

[elichai]

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)

[sipa]

@elichai Boost is C++. libsecp256k1 is C.

[elichai]

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