I was having only a version 1.3 of libtool and emerging flac-1.1.1 failed because the --tag option of libtool was not yet implemented. According to libtool changelog http://savannah.gnu.org/cgi-bin/viewcvs/libtool/libtool/ChangeLog?rev=1.1220.2.252&content-type=text/vnd.viewcvs-markup it was introduced in version 1.5.2. I don't know the details of ebuild files but I guess there is a dependency missing in it, to libtool-1.5.2. updating with "emerge libtool" first solved the problem Reproducible: Always Steps to Reproduce: 1. Having version 1.3 of libtool installed, 2. Attempt to emerge flac-1.1.1 Actual Results: libtool complains it does not know about that --tag option
If you have version 1.3, then you have an extremely outdated system. Current _stable_ version is 1.5.16. Update your system, please.