add CMake option tomlplusplus_INSTALL (#240)

Author: DapengFeng

CMakeLists.txt

@@ -31,8 +31,11 @@ target_include_directories(
 target_compile_features(tomlplusplus_tomlplusplus INTERFACE cxx_std_17)
 
 # ---- Install rules, examples, and fuzzing ----
-if(PROJECT_IS_TOP_LEVEL)
+if (tomlplusplus_INSTALL)
   include(cmake/install-rules.cmake)
+endif()
+
+if(PROJECT_IS_TOP_LEVEL)
   option(BUILD_EXAMPLES "Build examples tree." OFF)
   option(BUILD_FUZZER "Build fuzzer." OFF)
   if(BUILD_EXAMPLES)

cmake/variables.cmake

@@ -5,6 +5,7 @@
 # This is to provide a user experience similar to find_package when
 # add_subdirectory or FetchContent is used to consume this project
 set(tomlplusplus_warning_guard "")
+option(tomlplusplus_INSTALL "Enable generation of tomlplusplus install targets" ${PROJECT_IS_TOP_LEVEL})
 if(NOT PROJECT_IS_TOP_LEVEL)
   option(
       tomlplusplus_INCLUDES_WITH_SYSTEM