[Document] USE_OS_TZDB option in TZ installation document should be changed to USE_SYSTEM_TZ_DB

[Adamska1008]

Today I try to compile tz.cpp following this and find setting USE_OS_TZDB to on not work. I check the CMakeLists.txt and it turns out that that the actual option is USE_SYSTEM_TZ_DB.

[fkobi]

I also first thought that but it is an another option:

date/CMakeLists.txt

Lines 149 to 150 in 8a1102f

	if ( USE_SYSTEM_TZ_DB AND NOT WIN32 AND NOT MANUAL_TZ_DB )
	target_compile_definitions( date-tz PRIVATE INSTALL=. PUBLIC USE_OS_TZDB=1 )

Also USE_OS_TZDB is not recognized by cmake for me.