Fix build on GCC-based systems by adding the following to USES:
compiler:c++11-lang iconv
The former:
The compiler feature "cxx_decltype" is not known to CXX compiler "GNU" version 4.2.1.
The latter:
/usr/local/bin/ld: release/deps/libgettext_sys-b0dd51871dcd53eb.rlib(dcigettext.o): undefined reference to symbol 'libiconv'
Approved by: portmgr (tier-2 blanket)