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

examples/test-cmake/test-cmake.cpp

@@ -2,8 +2,12 @@
 #include <cstdio>
 
 int main(void) {
+#ifdef LLAMA_BUILD_NUMBER
     printf("[test-cmake] llama.cpp version: %s, build: %d (%s)\n",
            llama_version(), LLAMA_BUILD_NUMBER, LLAMA_BUILD_COMMIT);
+#else
+    printf("[test-cmake] llama.cpp version: %s\n", llama_version());
+#endif
     printf("[test-cmake] ggml version: %s, commit: %s\n", ggml_version(), ggml_commit());
     printf("[test-cmake] Initializing backend...\n");
     llama_backend_init();