[boost] [numeric][interval] clang support patch

Hi, I've been running into the (in-)famous clang bug http://llvm.org/bugs/show_bug.cgi?id=6907 which prevents detail/c99sub_rounding_control.hpp from working. Boost contains workaround code in <boost/detail/fenv.hpp> which is used by the patch attached. Ok to commit? Yours, Jürgen -- Dipl.-Math. Jürgen Hunold | IVE mbH Software-Entwickler | Lützerodestraße 10 Tel: +49 511 897668 33 | 30161 Hannover, Germany Fax: +49 511 897668 29 | http://www.ivembh.de juergen.hunold@ivembh.de | | Geschäftsführer: Sitz des Unternehmens: Hannover | Univ.-Prof. Dr.-Ing. Thomas Siefer Amtsgericht Hannover, HRB 56965 | PD Dr.-Ing. Alfons Radtke