build: Avoid secp256k1.h include from system #20469

While building i ran into an error because i had a version of secp256k1.h under /usr/local/include that was incompatible with the secp256k1 code in the repository. This caused a problem because $(BOOST_CPPFLAGS) contained -I/usr/local/include and the include paths are searched by the compiler in order from left to right, so in the end $(BITCOIN_INCLUDES) contained -I/usr/local/include before -I$(srcdir)/secp256k1/include which caused the compiler to find secp256k1.h under /usr/local/include.

Looking at git blame i am wondering how this has not happened to anyone else in several years: https://github.com/bitcoin/bitcoin/blame/cb89e18845852c005ba8ab94acc6544d7c876a42/src/Makefile.am#L25

I am on macOS 10.15.

This seems similar to #20351, but I think this is a cleaner solution than the (abandoned) fix from #20366.

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #18261 (Erlay: bandwidth-efficient transaction relay protocol by naumenkogs)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Nice minimal fix. Putting local include paths first, before those of dependencies makes a lot of sense.

Code review ACK e95aaefe2540cb76969818fcc2ff77d33448ed5a

Looking at git blame i am wondering how this has not happened to anyone else in several years:

Yes, I wonder too. But FWIW most people don't like to install dependencies system-wide manually, and there haven't been secp256k1 packages for most distributions. But it's definitely something that could trip up.

ACK e95aaefe2540cb76969818fcc2ff77d33448ed5a, tested on macOS 11 Big Sur by adding #error into /usr/local/include/secp256k1.h.

Backported in #20505.