feat(linking): control link mode of dependencies

[hamptokr]

This PR adds support for individually controlling dependencies' link mode static|dynamic via build.zig as requested in #7

Examples

    const libssh2_dependency = b.dependency("libssh2", .{
        .target = target,
        .optimize = optimize,
        .@"crypto-backend" = .mbedtls,
        .@"mbedtls-linkage" = .static,
    });
    exe.linkLibrary(libssh2_dependency.artifact("ssh2"));

Will statically link mbedtls, ldd output:

# ldd zig-out/bin/example
linux-vdso.so.1 (0x00007f3fe1e81000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f3fe1d42000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f3fe1a00000)
/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f3fe1e83000)

Whereas dynamic linking:

    const libssh2_dependency = b.dependency("libssh2", .{
        .target = target,
        .optimize = optimize,
        .@"crypto-backend" = .mbedtls,
        .@"mbedtls-linkage" = .dynamic,
    });
    exe.linkLibrary(libssh2_dependency.artifact("ssh2"));

Will result in:

# ldd zig-out/bin/example
linux-vdso.so.1 (0x00007fdf6a024000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007fdf69ee5000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007fdf69c00000)
/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007fdf6a026000)
libmbedcrypto.so.16 => /usr/lib/libmbedcrypto.so.16 (0x00007fdf69e54000)

On my machine statically linking (debug build) resulted in an 11mb binary. Dynamically linking reduced it to 8.3mb.