Document C++11 compiler requirement in BUILD.md

[Ryan Carsten Schmidt]

BUILD.md says:

re2c/BUILD.md

Line 8 in ece4bb7

 To build re2c from a release tarball one needs only a C++ compiler and Python 3 

But in fact one appears to need a C++11-compatible compiler; please document this.

[Ulya Trofimovich]

Thanks! Fixed in 7d66417.