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)

sherpa-onnx/jni/common.h

@@ -12,11 +12,31 @@
 #include "android/asset_manager_jni.h"
 #endif
 
+#if defined(_WIN32)
+#if defined(SHERPA_ONNX_BUILD_SHARED_LIBS)
+#define SHERPA_ONNX_EXPORT __declspec(dllexport)
+#define SHERPA_ONNX_IMPORT __declspec(dllimport)
+#else
+#define SHERPA_ONNX_EXPORT
+#define SHERPA_ONNX_IMPORT
+#endif
+#else  // WIN32
+#define SHERPA_ONNX_EXPORT __attribute__((visibility("default")))
+
+#define SHERPA_ONNX_IMPORT SHERPA_ONNX_EXPORT
+#endif  // WIN32
+
+#if defined(SHERPA_ONNX_BUILD_MAIN_LIB)
+#define SHERPA_ONNX_API SHERPA_ONNX_EXPORT
+#else
+#define SHERPA_ONNX_API SHERPA_ONNX_IMPORT
+#endif
+
 // If you use ndk, you can find "jni.h" inside
 // android-ndk/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/include
 #include "jni.h"  // NOLINT
 
-#define SHERPA_ONNX_EXTERN_C extern "C"
+#define SHERPA_ONNX_EXTERN_C extern "C" SHERPA_ONNX_API
 
 // defined in jni.cc
 jobject NewInteger(JNIEnv *env, int32_t value);