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

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

c-api-examples/CMakeLists.txt

@@ -1,6 +1,6 @@
 include(cargs)
 
-include_directories(${CMAKE_SOURCE_DIR})
+include_directories(${PROJECT_SOURCE_DIR})
 add_executable(decode-file-c-api decode-file-c-api.c)
 target_link_libraries(decode-file-c-api sherpa-onnx-c-api cargs)