refactor: restructure project to build a library for each book chapter

CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.20)
 
 project(
-  hello
+  book_exercises
   VERSION 0.1.0
   LANGUAGES CXX)
 
@@ -19,13 +19,17 @@ include(FetchContent)
 configure_file("${PROJECT_SOURCE_DIR}/include/version.h.in"
                "${PROJECT_BINARY_DIR}/include/version.h")
 
-add_executable(hello src/main.cpp src/null.cpp src/constexp.cpp
-                     src/ifswitch.cpp src/initlist.cpp)
+add_subdirectory(chapters)
 
-target_compile_features(hello PUBLIC cxx_std_20)
+add_executable(book_exercises src/main.cpp)
 
-target_include_directories(hello PRIVATE "${PROJECT_BINARY_DIR}/include"
-                                         "${PROJECT_SOURCE_DIR}/include")
+target_compile_features(book_exercises PUBLIC cxx_std_20)
+
+target_include_directories(
+  book_exercises PRIVATE "${PROJECT_BINARY_DIR}/include"
+                         "${PROJECT_SOURCE_DIR}/include")
+
+target_link_libraries(book_exercises PRIVATE chapter_02)
 
 if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
   add_subdirectory(tests)

chapters/CMakeLists.txt

@@ -0,0 +1 @@
+add_subdirectory(chapter_02)

chapters/chapter_02/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_library(chapter_02 src/initlist.cpp src/null.cpp src/constexp.cpp
+                       src/ifswitch.cpp)
+
+target_include_directories(chapter_02 PUBLIC include)