MFH: r443953

• Prevent asterisk build system from unconditionally using -march=native [1]
• Add an OPTIMIZED_CFLAGS option, disabled by default, to allow enabling the native flag
• While here, convert to option target helpers.

Reported by: "Andrey V. Elsukov" <bu7cher at yandex.ru> (via email) [1]

Approved by: ports-secteam (feld)