cmake : use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR (#28771)

This commit updates cmake to use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR for paths in function calls.

The motivation for this is that when using add_subdirectory,
CMAKE_SOURCE_DIR is fixed to the top-level projects source directory,
that is the caller of add_subdirectory and not the llama.cpp root
which means that common/common.h header will not be resolved.

Refs: https://github.com/ggml-org/llama.cpp/pull/28091#issuecomment-5636106377

.ecrc

@@ -1,5 +1,5 @@
 {
-  "Exclude": ["^\\.gitmodules$", "stb_image\\.h"],
+  "Exclude": ["^\\.gitmodules$", "stb_image\\.h", "examples/test-cmake/build/", "examples/test-cmake/build-subdir/"],
   "Disable": {
     "IndentSize": true
   }