libstdc++: Fix configuration of default zoneinfo dir on linux

The config for --with-libstdcxx-zoneinfo=yes was comparing the target
triplet to "gnu* | linux* | kfreebsd*-gnu | knetbsd*-gnu" which is only
the last component of the triplet, so failed to match and always used
the zoneinfo_dir=none default. Check $target_os instead.

There was also an error in the check for native builds that tzdata.zi is
actually present in the configured directory. That meant a warning was
printed even when the file was present:

configure: zoneinfo data directory: /usr/share/zoneinfo
configure: WARNING: "/usr/share/zoneinfo does not contain tzdata.zi file"
configure: static tzdata.zi file will be compiled into the library

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ZONEINFO_DIR): Check $target_os instead
	of $host. Fix check for file being present during native build.
	* configure: Regenerate.