BUILD_SHARED_LIBS output libSPIRV-Tools.a instead of libSPIRV-Tools.so

`BUILD_SHARED_LIBS` I think this option should lead to libSPIRV-Tools.so. but today I only find libSPIRV-Tools.a after compiling.
After searching log, I think maybe it's related to #3490
This is my cmake options
```
cmake \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_INSTALL_PREFIX=/usr \
-DCMAKE_INSTALL_LIBDIR=lib \
-DBUILD_SHARED_LIBS=ON \
-DSPIRV-Headers_SOURCE_DIR="${PWD}/../../spirv-headers" \
-DSPIRV_WERROR=OFF \
-SPIRV_TOOLS_EXTRA_DEFINITIONS=-DNDEBUG \
..
```