build.zig: change -Dpie option default to be target-dependant

`null` here indicates to Zig that it should decide for itself whether
to enable PIE or not. On some targets (like macOS or OpenBSD), it is
required and so current default will cause unneccessary build error.

Behavior for `-Dpie=false` and `-Dpie=true` is not changed.

Signed-off-by: Eric Joldasov <bratishkaerik@landless-city.net>

build.zig

@@ -7,21 +7,24 @@ pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    const pie = b.option(bool, "pie", "Build with PIE support (by default false)") orelse false;
+    const pie = b.option(bool, "pie", "Build with PIE support (by default: target-dependant)");
     const strip = b.option(bool, "strip", "Strip debugging info (by default false)") orelse false;
 
-    const exe = b.addExecutable(.{
+    const main_mod = b.createModule(.{
-        .name = "ncdu",
         .root_source_file = b.path("src/main.zig"),
         .target = target,
         .optimize = optimize,
         .strip = strip,
         .link_libc = true,
     });
+    main_mod.linkSystemLibrary("ncursesw", .{});
+    main_mod.linkSystemLibrary("zstd", .{});
 
+    const exe = b.addExecutable(.{
+        .name = "ncdu",
+        .root_module = main_mod,
+    });
     exe.pie = pie;
-    exe.root_module.linkSystemLibrary("ncursesw", .{});
-    exe.root_module.linkSystemLibrary("libzstd", .{});
     // https://github.com/ziglang/zig/blob/faccd79ca5debbe22fe168193b8de54393257604/build.zig#L745-L748
     if (target.result.os.tag.isDarwin()) {
         // useful for package maintainers
@@ -39,14 +42,9 @@ pub fn build(b: *std.Build) void {
     run_step.dependOn(&run_cmd.step);
 
     const unit_tests = b.addTest(.{
-        .root_source_file = b.path("src/main.zig"),
+        .root_module = main_mod,
-        .target = target,
-        .optimize = optimize,
-        .link_libc = true,
     });
     unit_tests.pie = pie;
-    unit_tests.root_module.linkSystemLibrary("ncursesw", .{});
-    unit_tests.root_module.linkSystemLibrary("libzstd", .{});
 
     const run_unit_tests = b.addRunArtifact(unit_tests);