CMakeLists.txt: Introduce ENABLE_PACKAGE_SOURCE option

This option makes the creation of 'package_source' custom target optional,
allowing to build with CMake < 3.11 if it is disabled (default).

CMakeLists.txt

@@ -20,7 +20,7 @@
 #
 ############################################################################
 
-cmake_minimum_required(VERSION 3.11) # we need CMake 3.11 for defining 'package_source' target as custom target
+cmake_minimum_required(VERSION 3.1)
 project(BELR VERSION 4.3.0 LANGUAGES C CXX)
 
 set(BELR_SO_VERSION "1")
@@ -30,6 +30,7 @@ option(ENABLE_STATIC "Build static library." ON)
 option(ENABLE_STRICT "Build with strict compile options." YES)
 option(ENABLE_TOOLS "Turn on or off compilation of tools." YES)
 option(ENABLE_TESTS "Enable compilation of unit tests." YES)
+option(ENABLE_PACKAGE_SOURCE "Create 'package_source' target for source archive making (CMake >= 3.11)" OFF)
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
@@ -151,4 +152,6 @@ install(FILES
 	DESTINATION ${ConfigPackageLocation}
 )
 
-add_subdirectory(build)
+if (ENABLE_PACKAGE_SOURCE)
+	add_subdirectory(build)
+endif()

build/CMakeLists.txt

@@ -20,6 +20,8 @@
 #
 ############################################################################
 
+cmake_minimum_required(VERSION 3.11) # we need CMake 3.11 for defining 'package_source' target as custom target
+
 if(NOT CPACK_PACKAGE_NAME)
 	set(CPACK_PACKAGE_NAME "belr")
 endif()