[tlbuild] dvisvgm fixed for Mac

[Richard M. Koch]

Martin Gieseking,

I applied the two patches you sent me to my copy of the TeX Live 2017 sources
and then built everything. There were no errors and all tests were passed. Thus
I think you have solved our problem. Many thanks.

Next year the TeX Live sources for the Mac will be built on system 10.10, Yosemite,
which is the earliest system still receiving security updates from Apple. For these
binaries we are in good shape.

Someone else will create binaries for earlier systems; the use of c++11 may complicate
that job, or require --disable-dvisvgm there.

Dick Koch