Out-of-source builds without environment variables.

Using CMake's configure_file command to generate a header file defining
a C string containing the path to the source root directory; the
directory where the resource folder is. Textures and models can now be
found directly as long the user doesn't move stuff around.

Shaders are still a problem.

CMakeLists.txt

@@ -111,6 +111,8 @@ set(5.advanced_lighting
 )
 
 
+configure_file(configuration/root_directory.h.in configuration/root_directory.h)
+include_directories(${CMAKE_BINARY_DIR}/configuration)
 
 foreach(CHAPTER ${CHAPTERS})
     foreach(DEMO ${${CHAPTER}})

configuration/root_directory.h.in

@@ -0,0 +1 @@
+const char * logl_root = "${CMAKE_SOURCE_DIR}";

includes/learnopengl/filesystem.h

@@ -2,6 +2,7 @@
 #define FILESYSTEM_H
 
 #include <string>
+#include "root_directory.h" // This is a configuration file generated by CMake.
 
 class FileSystem
 {
@@ -18,7 +19,8 @@ public:
 private:
   static std::string const & getRoot()
   {
-    static char const * givenRoot {getenv("LOGL_ROOT_PATH")};
+    static char const * envRoot = getenv("LOGL_ROOT_PATH");
+    static char const * givenRoot {envRoot != nullptr ? envRoot : logl_root};
     static std::string root {givenRoot != nullptr ? givenRoot : ""};
     return root;
   }