Update to 2.17.13.

• Do not complain about USE_GITHUB before USES [1]
• Make sure each variable in a dependency is fully expanded [2]
• Clean up the earlier checks for BROKEN, etc. to prevent bogus warnings [3]
• 'USE=execinfo' is removed with ports r450351. So stop suggesting to use it [4]

PR: 221971 [1]

		221605 [2]
		222409 [3]
		222653 [4]

Submitted by: amdmi3 [1]

		jrm [2]
		Yasuhiro KIMURA <yasu@utahime.org> [4]