define BOOST_TEST_DYN_LINK in CXXFLAGS rather than each test

This is a change to the tests only. The logic is that a developer may need to avoid `BOOST_TEST_DYN_LINK` from being defined and this change makes that easy to tweak in one location.

In my case I found that on OS X with boost 1.53 (and likely all recent versions) `BOOST_TEST_DYN_LINK` and a static `libboost_unit_test_framework.a` do not play well together. Linking to a dynamic library works, but a static library results in:

```sh
Checking t/geometry/test_linestring_geometry.cpp...
g++ -I../include -I. -g -Wall -Wextra -Wredundant-decls -Wdisabled-optimization -pedantic -Wctor-dtor-privacy -Wnon-virtual-dtor -Woverloaded-virtual -Wsign-promo -o tests -I/opt/boost-53/include test_main.cpp test_utils.cpp t/geometry/test_linestring_geometry.cpp -L/opt/boost-53/lib/ -lboost_unit_test_framework
Undefined symbols for architecture x86_64:
  "boost::unit_test::unit_test_main(bool (*)(), int, char**)", referenced from:
      _main in cc8zROUk.o
ld: symbol(s) not found for architecture x86_64
```

However linking a static `libboost_unit_test_framework.a` works just fine as long as `BOOST_TEST_DYN_LINK`  is not defined.