Use system integration options API

[RokasPuzonas]

Resolves #7

Here is an example for how the build.zig needs to look if you are using -fno-sys=mbedtls:

const mbedtls_dependency = b.dependency("mbedtls", .{
    .target = target,
    .optimize = optimize,
});

const libssh2_dependency = b.dependency("libssh2", .{
    .target = target,
    .optimize = optimize,
    .strip = optimize != .Debug,
    .@"crypto-backend" = .mbedtls,
});
const libssh2_artifact = libssh2_dependency.artifact("ssh2");
 
// This line is very important, it's important to link mbedtls with libssh2. Not with root_module.
// This is a very easy mistake to make.
libssh2_artifact.linkLibrary(mbedtls_dependency.artifact("mbedtls"));

root_module.linkLibrary(libssh2_artifact);

const exe = b.addExecutable(.{
    .name = "resource_tracker",
    .root_module = root_module
});

[hamptokr]

System Integration Options do not control whether something is statically or dynamically linked, it's just controlling whether or not we should use the system library. That said, your changes below do make sense as it allows the user to provide/link their own vendored version if they want while still providing a sane default where it "just works"

[hamptokr]

As mentioned, this has nothing to do with static/dynamic linking just means we don't try to link the system library for folks automatically

We can provide an error message here maybe:

Suggested change

	// TODO: Add lazy dependency to build.zig.zon for linking statically.
	// For now it's the users resposibility to compile and statically link against library
	@panic("mbedtls not provided. If you pass --no-sys=mbedtls you must link your own mbedtls to the final app.");

[RokasPuzonas]

I'm not sure if a nice error message can be provided, because using @panic here will make it always show the message.

Currently the error you will get when it isn't linked is this:

/home/rokas/.cache/zig/p/N-V-__8AAATRLQBu_rNy4X2UK6RtcdYK_yAzkV6OcCqgo5aK/src/mbedtls.h:59:10: error: 'mbedtls/version.h' file not found
#include <mbedtls/version.h>
         ^~~~~~~~~~~~~~~~~~~~
/home/rokas/.cache/zig/p/N-V-__8AAATRLQBu_rNy4X2UK6RtcdYK_yAzkV6OcCqgo5aK/src/agent.c:42:10: note: in file included from /home/rokas/.cache/zig/p/N-V-__8AAATRLQBu_rNy4X2UK6RtcdYK_yAzkV6OcCqgo5aK/src/agent.c:42:
#include "libssh2_priv.h"
         ^
/home/rokas/.cache/zig/p/N-V-__8AAATRLQBu_rNy4X2UK6RtcdYK_yAzkV6OcCqgo5aK/src/libssh2_priv.h:187:10: note: in file included from /home/rokas/.cache/zig/p/N-V-__8AAATRLQBu_rNy4X2UK6RtcdYK_yAzkV6OcCqgo5aK/src/libssh2_priv.h:187:
#include "crypto.h"
         ^
/home/rokas/.cache/zig/p/N-V-__8AAATRLQBu_rNy4X2UK6RtcdYK_yAzkV6OcCqgo5aK/src/crypto.h:49:10: note: in file included from /home/rokas/.cache/zig/p/N-V-__8AAATRLQBu_rNy4X2UK6RtcdYK_yAzkV6OcCqgo5aK/src/crypto.h:49:
#include "mbedtls.h"

// ...

[hamptokr]

Same here, let's add an error message

[hamptokr]

Might as well use linkSystemLibrary since we aren't passing any options

[hamptokr]

Same here, add an error message