Update Zig requirement in README + set preferred build mode

+ minor irrelevant build system changes.

Makefile

@@ -9,7 +9,7 @@ ZIG ?= zig
 PREFIX ?= /usr/local
 BINDIR ?= ${PREFIX}/bin
 MANDIR ?= ${PREFIX}/share/man/man1
-ZIG_FLAGS ?= -Doptimize=ReleaseFast
+ZIG_FLAGS ?= --release
 
 NCDU_VERSION=$(shell grep 'program_version = "' src/main.zig | sed -e 's/^.*"\(.\+\)".*$$/\1/')
 
@@ -65,7 +65,7 @@ static-%.tar.gz:
 		CC="${ZIG} cc --target=$*"\
 		LD="${ZIG} cc --target=$*"\
 		AR="${ZIG} ar" RANLIB="${ZIG} ranlib"\
-		CPPFLAGS=-D_GNU_SOURCE && make && make install.libs
+		CPPFLAGS=-D_GNU_SOURCE && make -j8 && make install.libs
 	@# zig-build - cleaner approach but doesn't work, results in a dynamically linked binary.
 	@#cd static-$* && PKG_CONFIG_LIBDIR="`pwd`/inst/pkg" zig build -Dtarget=$*
 	@#	--build-file ../build.zig --search-prefix inst/ --cache-dir zig -Drelease-fast=true

README.md

@@ -19,7 +19,7 @@ C version (1.x).
 
 ## Requirements
 
-- Zig 0.11.0
+- Zig 0.12.0
 - Some sort of POSIX-like OS
 - ncurses libraries and header files
 

build.zig

@@ -5,9 +5,9 @@ const std = @import("std");
 
 pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
-    const t = target.result;
-
-    const optimize = b.standardOptimizeOption(.{});
+    const optimize = b.standardOptimizeOption(.{
+        .preferred_optimize_mode = .ReleaseFast,
+    });
 
     const pie = b.option(bool, "pie", "Build with PIE support (by default false)") orelse false;
 
@@ -22,7 +22,7 @@ pub fn build(b: *std.Build) void {
     exe.pie = pie;
     exe.root_module.linkSystemLibrary("ncursesw", .{});
     // https://github.com/ziglang/zig/blob/b52be973dfb7d1408218b8e75800a2da3dc69108/build.zig#L551-L554
-    if (t.isDarwin()) {
+    if (target.result.isDarwin()) {
         // useful for package maintainers
         exe.headerpad_max_install_names = true;
     }