harfbuzz fails on netbsd-7 i386

[Greg Troxel <gdt%lexort.com@localhost>]

On netbsd-7 i386, harfbuzz fails with some really hard to follow C++
errors on two files.  This is using gcc 4.8.5.  It's not documented to
need C++11, and there is no GCC_REQD.

Adding GCC_REQD+=6 made it build.  I picked 6 because the box in
question already had it built.

Perhaps this error will turn up in the forthcoming bulk builds.