Support for out-of-source builds.

Uses environment variable to tell the program where to find resource
files.
Sharder sources are still search for in the current workind directory.

includes/learnopengl/filesystem.h

@@ -0,0 +1,49 @@
+#ifndef FILESYSTEM_H
+#define FILESYSTEM_H
+
+#include <string>
+
+class FileSystem
+{
+private:
+  typedef std::string (*Builder) (const std::string& path);
+
+public:
+  static std::string getPath(const std::string& path)
+  {
+    static std::string(*pathBuilder)(std::string const &) = getPathBuilder();
+    return (*pathBuilder)(path);
+  }
+
+private:
+  static std::string const & getRoot()
+  {
+    static char const * givenRoot {getenv("LOGL_ROOT_PATH")};
+    static std::string root {givenRoot != nullptr ? givenRoot : ""};
+    return root;
+  }
+
+  //static std::string(*foo (std::string const &)) getPathBuilder()
+  static Builder getPathBuilder()
+  {
+    if (getRoot() != "")
+      return &FileSystem::getPathRelativeRoot;
+    else
+      return &FileSystem::getPathRelativeBinary;
+  }
+
+  static std::string getPathRelativeRoot(const std::string& path)
+  {
+    return getRoot() + std::string("/") + path;
+  }
+
+  static std::string getPathRelativeBinary(const std::string& path)
+  {
+    return "../../../" + path;
+  }
+
+
+};
+
+// FILESYSTEM_H
+#endif