build: derive default config path from install prefix

- Add `default_config_path` build option for the river executable

 build.zig      | 16 ++++++++++------
 river/main.zig |  4 +++-
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/build.zig b/build.zig
index 62f4afc..494f0ce 100644
--- a/build.zig
+++ b/build.zig
@@ -27,13 +27,16 @@ pub fn build(b: *zbs.Builder) !void {
 
     const examples = b.option(bool, "examples", "Set to true to build examples") orelse false;
 
-    // Sigh, why are the conventions inconsistent like this.
     const resolved_prefix = try std.fs.path.resolve(b.allocator, &[_][]const u8{b.install_prefix.?});
-    if (std.mem.eql(u8, resolved_prefix, "/usr")) {
-        b.installFile("example/init", "../etc/river/init");
-    } else {
-        b.installFile("example/init", "etc/river/init");
-    }
+    const rel_config_path = if (std.mem.eql(u8, resolved_prefix, "/usr"))
+        "../etc/river/init"
+    else
+        "etc/river/init";
+    b.installFile("example/init", rel_config_path);
+    const default_config_path = try std.fs.path.resolve(
+        b.allocator,
+        &[_][]const u8{ resolved_prefix, rel_config_path },
+    );
 
     const scanner = ScanProtocolsStep.create(b);
     scanner.addSystemProtocol("stable/xdg-shell/xdg-shell.xml");
@@ -47,6 +50,7 @@ pub fn build(b: *zbs.Builder) !void {
         river.setTarget(target);
         river.setBuildMode(mode);
         river.addBuildOption(bool, "xwayland", xwayland);
+        river.addBuildOption([]const u8, "default_config_path", default_config_path);
 
         addServerDeps(river, scanner);
 
diff --git a/river/main.zig b/river/main.zig
index d576bb2..635e30e 100644
--- a/river/main.zig
+++ b/river/main.zig
@@ -18,6 +18,8 @@
 const std = @import("std");
 const wlr = @import("wlroots");
 
+const build_options = @import("build_options");
+
 const c = @import("c.zig");
 const log = @import("log.zig");
 const util = @import("util.zig");
@@ -52,7 +54,7 @@ fn getStartupCommand() std.fmt.AllocPrintError!?[:0]const u8 {
             return path;
         }
     }
-    if (try testConfigPath("/etc/river/init", .{})) |path| {
+    if (try testConfigPath(build_options.default_config_path, .{})) |path| {
         return path;
     }
     return null;