Build sherpa-onnx as a single shared library (#1078)

When `-D BUILD_SHARED_LIBS=ON` is passed to `cmake`, it builds a single shared library.

Specifically, 

- For C APIs, it builds `libsherpa-onnx-c-api.so`
- For Python APIs, it builds `_sherpa_onnx.cpython-xx-xx.so`
- For Kotlin and Java APIs, it builds `libsherpa-onnx-jni.so`

There is no `libsherpa-onnx-core.so` any longer.

Note it affects only shared libraries.

sherpa-onnx/jni/CMakeLists.txt

@@ -35,5 +35,8 @@ endif()
 
 add_library(sherpa-onnx-jni ${sources})
 
+target_compile_definitions(sherpa-onnx-jni PRIVATE SHERPA_ONNX_BUILD_SHARED_LIBS=1)
+target_compile_definitions(sherpa-onnx-jni PRIVATE SHERPA_ONNX_BUILD_MAIN_LIB=1)
+
 target_link_libraries(sherpa-onnx-jni sherpa-onnx-core)
 install(TARGETS sherpa-onnx-jni DESTINATION lib)