Do not build converters/libiconv port on FreeBSD 10 and higher.  libiconv was imported
into base system on FreeBSD 10, and converters/libiconv does not build on FreeBSD 10.
Do not build sysutils/fusefs-kmod on FreeBSD 10 and higher.
In FreeBSD 10, fusefs was kernel module was made part of base system
and sysutils/fusefs-kmod does not build.
Native FreeBSD NTFS implementation and mount_ntfs binary was removed in FreeBSD 10, so no need
to rename mount_ntfs binary on FreeBSD 10 and higher.
Do not use gcc47 if we are using FreeBSD 10 or higher, where we instead use clang.
Add tweak to make.conf.install to allow installing libc++.so library from clang
into $NANO_OBJ/_.w staging environment when using FreeBSD 10.