Fixes #449 by adding explicit keyword to constructor

In an ideal scenario I would write a fails to compile test. I looked for a nice way to do it with SNIAFE but it's been flaky with me in the past and I've forgotten how to.

I have considered adding try_compile to the CMake, something like:

try_compile(TEST_NAME, "TEST_NAME_File.cpp")
# ... more tests ...
configure_file("CompileTests.cpp.in")

where CompileTests.cpp.in contains something like:

constexpr bool TEST_NAMECompiles = @TEST_NAME@;
// ... and so on for all tests ... 
TEST(CompileTests, TEST_NAME) 
{
    ASSERT_FALSE(TEST_NAMECompiles);
   // Personally I wouldn't add any `ASSERT_TRUE` tests like this
}

I thought GDAL did something similar but I had a look at the source code and can't see it so I assume I've confused it with something (I had a look at geos too but nothing obvious).