build: introduce ENABLE_DATE_INSTALL to allow user opt-out of install

When using `date` as a PRIVATE dependency, it is not required to install it. This flag give user using `date` with add_subdirectory the opportunity to disable install target Default behavior is conserved since ENABLE_DATE_INSTALL is ON

CMakeLists.txt

@@ -20,8 +20,6 @@ cmake_minimum_required( VERSION 3.7 )
 project( date VERSION 3.0.1 )
 set(ABI_VERSION 3) # used as SOVERSION, increment when ABI changes
 
-include( GNUInstallDirs )
-
 get_directory_property( has_parent PARENT_DIRECTORY )
 
 if (POLICY CMP0077)
@@ -40,12 +38,17 @@ option( ENABLE_DATE_TESTING "Enable unit tests" OFF )
 option( DISABLE_STRING_VIEW "Disable string view" OFF )
 option( COMPILE_WITH_C_LOCALE "define ONLY_C_LOCALE=1" OFF )
 option( BUILD_TZ_LIB "build/install of TZ library" OFF )
+option( ENABLE_DATE_INSTALL "Enable unit tests" ON )
 
 if( ENABLE_DATE_TESTING AND NOT BUILD_TZ_LIB )
     message(WARNING "Testing requested, but BUILD_TZ_LIB not ON - forcing the latter")
     set (BUILD_TZ_LIB ON CACHE BOOL "required for testing" FORCE)
 endif( )
 
+if( ENABLE_DATE_INSTALL )
+  include( GNUInstallDirs )
+endif( )
+
 function( print_option OPT )
     if ( NOT DEFINED PRINT_OPTION_CURR_${OPT} OR ( NOT PRINT_OPTION_CURR_${OPT} STREQUAL ${OPT} ) )
         set( PRINT_OPTION_CURR_${OPT} ${${OPT}} CACHE BOOL "" )
@@ -168,6 +171,7 @@ endif( )
 #[===================================================================[
    installation
 #]===================================================================]
+if( ENABLE_DATE_INSTALL )
   set( version_config "${CMAKE_CURRENT_BINARY_DIR}/dateConfigVersion.cmake" )
 
   include( CMakePackageConfigHelpers )
@@ -207,6 +211,7 @@ install( EXPORT dateConfig
   install (
     FILES cmake/dateConfig.cmake "${version_config}"
     DESTINATION ${CONFIG_LOC})
+endif( )
 
 #[===================================================================[
    testing