Bind xtal_tdzdd as static library

Shared libraries are localted under site-packages which is not usually
in LD_LIBRARY_PATH. Thus, this commit makes internal `xtal_tdzdd` as
static library and position independent.
See pybind/cmake_example#11 (comment)

CMakeLists.txt

@@ -31,4 +31,4 @@ message(STATUS ${pybind11_SOURCE_DIR})
 
 add_subdirectory(${pybind11_SOURCE_DIR})
 pybind11_add_module(pyzdd_ ./xtal_tdzdd/wrapper.cpp)
-target_link_libraries(pyzdd_ PUBLIC xtal_tdzdd)
+target_link_libraries(pyzdd_ PRIVATE xtal_tdzdd)

xtal_tdzdd/CMakeLists.txt

@@ -4,7 +4,6 @@ project(xtal_tdzdd CXX)
 
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED True)
-set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 
 # Fetch TdZdd
 include(FetchContent)
@@ -23,7 +22,8 @@ endif()
 
 add_compile_options(-O3 -Wall -Winit-self -Wlogical-op)
 
-add_library(xtal_tdzdd SHARED
+add_library(xtal_tdzdd STATIC
             iterator.cpp)
+set_target_properties(xtal_tdzdd PROPERTIES POSITION_INDEPENDENT_CODE ON)
 target_include_directories(xtal_tdzdd PRIVATE ./)
 target_include_directories(xtal_tdzdd PRIVATE ${tdzdd_SOURCE_DIR}/include)