zig build: adjust DESTDIR logic

now if DESTDIR is provided then the default install prefix is /usr.
otherwise the default install prefix is still zig-cache directly.

DESTDIR is prepended to the prefix to match what make install does.

std/build.zig

@@ -44,6 +44,7 @@ pub const Builder = struct {
     dest_dir: ?[]const u8,
     lib_dir: ?[]const u8,
     exe_dir: ?[]const u8,
+    install_path: []const u8,
     search_prefixes: ArrayList([]const u8),
     installed_files: ArrayList(InstalledFile),
     build_root: []const u8,
@@ -144,6 +145,7 @@ pub const Builder = struct {
             .is_release = false,
             .override_std_dir = null,
             .override_lib_dir = null,
+            .install_path = undefined,
         };
         try self.top_level_steps.append(&self.install_tls);
         try self.top_level_steps.append(&self.uninstall_tls);
@@ -166,14 +168,19 @@ pub const Builder = struct {
     }
 
     fn resolveInstallPrefix(self: *Builder) void {
-        const dest_dir = self.dest_dir orelse blk: {
-            const dest_dir = self.install_prefix orelse self.cache_root;
-            self.dest_dir = dest_dir;
-            break :blk dest_dir;
-        };
-        self.dest_dir = dest_dir;
-        self.lib_dir = fs.path.join(self.allocator, [_][]const u8{ dest_dir, "lib" }) catch unreachable;
-        self.exe_dir = fs.path.join(self.allocator, [_][]const u8{ dest_dir, "bin" }) catch unreachable;
+        if (self.dest_dir) |dest_dir| {
+            const install_prefix = self.install_prefix orelse "/usr";
+            self.install_path = fs.path.join(self.allocator, [_][]const u8{ dest_dir, install_prefix }) catch unreachable;
+        } else {
+            const install_prefix = self.install_prefix orelse blk: {
+                const p = self.cache_root;
+                self.install_prefix = p;
+                break :blk p;
+            };
+            self.install_path = install_prefix;
+        }
+        self.lib_dir = fs.path.join(self.allocator, [_][]const u8{ self.install_path, "lib" }) catch unreachable;
+        self.exe_dir = fs.path.join(self.allocator, [_][]const u8{ self.install_path, "bin" }) catch unreachable;
     }
 
     pub fn addExecutable(self: *Builder, name: []const u8, root_src: ?[]const u8) *LibExeObjStep {
@@ -886,7 +893,7 @@ pub const Builder = struct {
 
     fn getInstallPath(self: *Builder, dir: InstallDir, dest_rel_path: []const u8) []const u8 {
         const base_dir = switch (dir) {
-            .Prefix => self.dest_dir.?,
+            .Prefix => self.install_path,
             .Bin => self.exe_dir.?,
             .Lib => self.lib_dir.?,
         };