Allow building repository as CMake subdirectory (#2059)

* Use PROJECT_SOURCE_DIR rather than CMAKE_SOURCE_DIR to allow building as subdirectory

* Also use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR in c/cxx api examples

* Only build examples by default when not building as subdirectory

* Do not suggest building binaries either

---------

Co-authored-by: user <user@mail.tld>

sherpa-onnx/csrc/CMakeLists.txt

@@ -1,4 +1,4 @@
-include_directories(${CMAKE_SOURCE_DIR})
+include_directories(${PROJECT_SOURCE_DIR})
 
 if(SHERPA_ONNX_ENABLE_PYTHON)
   message(STATUS "PYTHON_EXECUTABLE: ${PYTHON_EXECUTABLE}")
@@ -648,7 +648,7 @@ endforeach()
 # For clang-tidy
 add_custom_target(
   clang-tidy-check
-  clang-tidy -p ${CMAKE_BINARY_DIR}/compile_commands.json --config-file ${CMAKE_SOURCE_DIR}/.clang-tidy ${srcs_to_check}
+  clang-tidy -p ${CMAKE_BINARY_DIR}/compile_commands.json --config-file ${PROJECT_SOURCE_DIR}/.clang-tidy ${srcs_to_check}
   DEPENDS ${sources})
 
 add_custom_target(check DEPENDS clang-tidy-check)