Change contrib/laxder from headers-only to files compilable as standalone C #343

Verified that both programs compile with

gcc -I. -I../include -lsecp256k1 -c -W -Wextra -Wall -Werror -ansi -pedantic lax_der_privatekey_parsing.c
gcc -I. -I../include -lsecp256k1 -c -W -Wextra -Wall -Werror -ansi -pedantic lax_der_parsing.c

Eh, I don't like this. These things are not compilable modules with a neat exposed API, they're just pieces of example code...

I don't see where the modules comment comes from? They're now just ordinary C code, not weird header stuff. They're still just code fragments, not libraries, not modules, not added to the build system, etc.

Oh you're referring the linking into libsecp256k1? Yea, I don't agree with that, but this patch doesn't do that.

Would be good if it touched the build system enough to fix tests.c, however!

Turning it into full .c files with corresponding header files gives a pretty strong impression that this is code that is to be compiled and linked into your application as-is.

I really want to discourage that. You shouldn't be compiling code from another project's source tree. If that was the intention, we should just make it a compile time flag and expose it through the library.

This is an example code snippet you can copy-paste into your application or translate to another language as needed, making changes as needed too.

Updated to fix tests.c, add warning for users not to link directly from our codebase, and remove the secp256k1_ prefixes from the functions to emphasize that they are not part of libsecp256k1 proper.

You need to add the .c files to the makefiles.

@apoelstra Ping?

I added the .c files to noinst_HEADERS. Is this right?

It seems the almighty Travis is happy with it.