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.

cmake/kaldi-decoder.cmake

@@ -45,8 +45,25 @@ function(download_kaldi_decoder)
   message(STATUS "kaldi-decoder's binary dir is ${kaldi_decoder_BINARY_DIR}")
 
   include_directories(${kaldi_decoder_SOURCE_DIR})
+
+  if(BUILD_SHARED_LIBS)
+    set(_build_shared_libs_bak ${BUILD_SHARED_LIBS})
+    set(BUILD_SHARED_LIBS OFF)
+  endif()
+
   add_subdirectory(${kaldi_decoder_SOURCE_DIR} ${kaldi_decoder_BINARY_DIR} EXCLUDE_FROM_ALL)
 
+  if(_build_shared_libs_bak)
+    set_target_properties(
+        kaldi-decoder-core
+      PROPERTIES
+        POSITION_INDEPENDENT_CODE ON
+        C_VISIBILITY_PRESET hidden
+        CXX_VISIBILITY_PRESET hidden
+    )
+    set(BUILD_SHARED_LIBS ON)
+  endif()
+
   if(WIN32 AND MSVC)
     target_compile_options(kaldi-decoder-core PUBLIC
       /wd4018
@@ -58,14 +75,7 @@ function(download_kaldi_decoder)
     INTERFACE
       ${kaldi-decoder_SOURCE_DIR}/
   )
-  if(SHERPA_ONNX_ENABLE_PYTHON AND WIN32)
-    install(TARGETS
-      kaldi-decoder-core
-      kaldifst_core
-      fst
-      fstfar
-    DESTINATION ..)
-  else()
+  if(NOT BUILD_SHARED_LIBS)
     install(TARGETS
       kaldi-decoder-core
       kaldifst_core
@@ -73,15 +83,6 @@ function(download_kaldi_decoder)
       fstfar
     DESTINATION lib)
   endif()
-
-  if(WIN32 AND BUILD_SHARED_LIBS)
-    install(TARGETS
-      kaldi-decoder-core
-      kaldifst_core
-      fst
-      fstfar
-    DESTINATION bin)
-  endif()
 endfunction()
 
 download_kaldi_decoder()