[tlbuild] failure to propagate TL_CONFIGURE_ARGS completely

[Karl Berry]

Hi Nelson - back on this report from 2016 (sorry):
https://tug.org/pipermail/tlbuild/2016q2/003548.html

    However, that option [--build] supplied in the environment like this:
             env	TL_MAKE=/usr/local/bin/gmake \
                    TL_CONFIGURE_ARGS=--build=x86_64-unknown-freebsd10.2 \
                    texlive-20160504/source/Build
    fails to propagate to the freetype2 configure script:

I changed freetype2/configure.ac to pass a --build argument to the
freetype (sub)configure if specified (r49476). Before, --build was only
passed if --host was also specified.

It seems to me that in principle all arguments from configure should be
passed, but I'm guessing peb had a good reason to do it this way, so
went with the minimal change.

I could not find any other setups with this particular problem, though
I certainly could have missed it.

Let me know if this problem persists ... --thanks, karl.