Factor out directory finding from prefix detection

This will also be used to locate the `yabridge.tmol` configuration file.

src/common/configuration.h

@@ -0,0 +1,52 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020  Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#pragma once
+
+#include <filesystem>
+#include <optional>
+
+/**
+ * Starting from the starting file or directory, go up in the directory
+ * hierarchy until we find a file named `filename`.
+ *
+ * @param filename The name of the file we're looking for. This can also be a
+ *   directory name since directories are also files.
+ * @param starting_from The directory to start searching in. If this is a file,
+ *   then start searching in the directory the file is located in.
+ * @param predicate The predicate to use to check if the path matches a file.
+ *   Needed as an easy way to limit the search to directories only since C++17
+ *   does not have any built in coroutines or generators.
+ *
+ * @return The path to the *file* found, or `std::nullopt` if the file could not
+ *   be found.
+ */
+template <typename F = bool(const std::filesystem::path&)>
+std::optional<std::filesystem::path> find_dominating_file(
+    const std::string& filename,
+    std::filesystem::path starting_dir,
+    F predicate = std::filesystem::exists) {
+    while (starting_dir != "") {
+        const std::filesystem::path candidate = starting_dir / filename;
+        if (predicate(candidate)) {
+            return candidate;
+        }
+
+        starting_dir = starting_dir.parent_path();
+    }
+
+    return std::nullopt;
+}