Do not merge this.
https://github.com/bitcoin-core/secp256k1/pull/787 moves endianness detection from autotools to the C code, which seems doable on modern platforms, and simplifies getting things right on autotools-less environments.
As libsecp256k1 doesn't (yet) have MSVC-based ci, I'm (ab)using Bitcoin Core's for this change.
5e5fb28b4a Use additional system macros to figure out endianness
979961c506 Merge #787: Use preprocessor macros instead of autoconf to detect endianness
887bd1f8b6 Merge #793: Make scalar/field choice depend on C-detected __int128 availability
0dccf98a21 Use preprocessor macros instead of autoconf to detect endianness
b2c8c42cf1 Merge #795: Avoid linking libcrypto in the valgrind ct test.
57d3a3c64c Avoid linking libcrypto in the valgrind ct test.
79f1f7a4f1 Autodetect __int128 availability on the C side
0d7727f95e Add SECP256K1_FE_STORAGE_CONST_GET to 5x52 field
805082de11 Merge #696: Run a Travis test on s390x (big endian)
39295362cf Test travis s390x (big endian)
6034a04fb1 Merge #778: secp256k1_gej_double_nonzero supports infinity
f60915906d Merge #779: travis: Fix argument quoting for ./configure
9e49a9b255 travis: Fix argument quoting for ./configure
18d36327fd secp256k1_gej_double_nonzero supports infinity
214cb3c321 Merge #772: Improve constant-timeness on PowerPC
40412b1930 Merge #774: tests: Abort if malloc() fails during context cloning tests
2e1b9e0458 tests: Abort if malloc() fails during context cloning tests
67a429f31f Suppress a harmless variable-time optimization by clang in _int_cmov
5b196338f0 Remove redundant "? 1 : 0" after comparisons in scalar code
3e5cfc5c73 Merge #741: Remove unnecessary sign variable from wnaf_const
66bb9320c0 Merge #773: Fix some compile problems on weird/old compilers.
1309c03c45 Fix some compile problems on weird/old compilers.
2309c7dd4a Merge #769: Undef HAVE___INT128 in basic-config.h to fix gen_context compilation
22e578bb11 Undef HAVE___INT128 in basic-config.h to fix gen_context compilation
3f4a5a10e4 Merge #765: remove dead store in ecdsa_signature_parse_der_lax
f00d6575ca remove dead store in ecdsa_signature_parse_der_lax
dbd41db16a Merge #759: Fix uninitialized variables in ecmult_multi test
2e7fc5b537 Fix uninitialized variables in ecmult_multi test
37dba329c6 Remove unnecessary sign variable from wnaf_const
6bb0b77e15 Fix test_constant_wnaf for -1 and add a test for it.
git-subtree-dir: src/secp256k1
git-subtree-split: 5e5fb28b4a45d7e35e55b5f5feead2be07bccc28
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