[tlbuild] ICU build error on FreeBSD 11.1

[Karl Berry]

    https://svnweb.freebsd.org/ports/head/devel/icu-le-hb/Makefile?r1=461290&r2=461289&pathrev=461290

Just guessing from the diff there, it would seem they are requiring
-std=gnu++11 instead of just -std=c++11 (the latter appears in your
compile command).  If this is truly required (seems improbable, since
ICU would surely not intentionally require GNU C++ features), it seems
strange that this has not come up on other platforms.

Nikola, can you shed any light? Have you had a chance to try compiling
the current sources? --thanks, karl.