Fix command-line-tools-only build on macOS

[Gnimuc]

I screwed this up in #406 when I was trying to fix a mysterious failure on Travis CI.

[dpo]

Many thanks!

Test Summary:                   | Pass  Total
import without using Cxx (#316) |    2      2
foo
foo
foo
foo
foo
foo
foo
foo
foo
foo
1
2
3
4
5
6
7
8
9
10
Test Summary: | Pass  Total
StdString     |    3      3
Test Summary: | Pass  Total
StdVector     |   70     70
e = (class std::length_error &) {
}

Test Summary: | Pass  Total
Exceptions    |    2      2
   Testing Cxx tests passed