Enable testing in CMakeLists.txt

* Per instructions by @SlavSlavov in https://github.com/HowardHinnant/date/pull/278

CMakeLists.txt

@@ -4,11 +4,10 @@ project( date_prj )
 
 find_package( Threads REQUIRED )
 
-enable_testing( )
-
 option( USE_SYSTEM_TZ_DB "Use the operating system's timezone database" OFF )
 option( USE_TZ_DB_IN_DOT "Save the timezone database in the current folder" OFF )
 option( BUILD_TZ_STATIC  "Build a static version of library" ON )
+option( ENABLE_DATE_TESTING "Enable unit tests" ON )
 
 function( print_option OPT )
 	if ( NOT DEFINED PRINT_OPTION_CURR_${OPT} OR ( NOT PRINT_OPTION_CURR_${OPT} STREQUAL ${OPT} ) )
@@ -21,6 +20,7 @@ endfunction( )
 print_option( USE_SYSTEM_TZ_DB )
 print_option( USE_TZ_DB_IN_DOT )
 print_option( BUILD_TZ_STATIC  )
+print_option( ENABLE_DATE_TESTING )
 
 set( HEADER_FOLDER "include" )
 set( SOURCE_FOLDER "src" )
@@ -100,6 +100,10 @@ install( EXPORT DateConfig DESTINATION ${DEF_INSTALL_CMAKE_DIR} )
 install( TARGETS tz DESTINATION lib )
 install( DIRECTORY ${HEADER_FOLDER}/ DESTINATION include/ )
 
+if ( ENABLE_DATE_TESTING )
+
+    enable_testing( )
+
     add_custom_target( testit COMMAND ${CMAKE_CTEST_COMMAND} )
 
     add_dependencies( testit tz )
@@ -159,4 +163,4 @@ foreach( child ${children} )
             endif( )
         endif( )
     endforeach( )
-
+endif( )