Building tng as a subproject in a subdirectory

[madebr]

Hello!

Recently, we received a package inclusion request for tng 1.8.2 at conan-center-index.
It appears tng cannot build correctly when used as a subproject.
This can be fixed by applying the following patch:

--- BuildTNG.cmake
+++ BuildTNG.cmake
@@ -1,9 +1,9 @@
 set(TNG_ROOT_SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR})
-file(RELATIVE_PATH TNG_ROOT_BINARY_DIR ${CMAKE_SOURCE_DIR} ${TNG_ROOT_SOURCE_DIR})
+file(RELATIVE_PATH TNG_ROOT_BINARY_DIR ${PROJECT_SOURCE_DIR} ${TNG_ROOT_SOURCE_DIR})
 if ("${TNG_ROOT_BINARY_DIR}" MATCHES "^\.\.")
     set(TNG_ROOT_BINARY_DIR tng)
 endif()
-set(TNG_ROOT_BINARY_DIR ${CMAKE_BINARY_DIR}/${TNG_ROOT_BINARY_DIR})
+set(TNG_ROOT_BINARY_DIR ${PROJECT_BINARY_DIR}/${TNG_ROOT_BINARY_DIR})
 
 set(TNG_MAJOR_VERSION "1")
 set(TNG_MINOR_VERSION "8")

I believe this patch would also be useful for the tng library itself.