This makes it easier to use GTest in projects that build with the
-Wmissing-declarations warning. This fixes the warning in headers and
source files, though not GTest's own tests as it is rather noisy there.
As mentioned in issue #360:
"Now that all the platforms gtest supports work with value-parameterized
tests, we should remove the uses of the GTEST_HAS_PARAM_TESTS macro from
the codebase everywhere."
https://github.com/google/googletest/issues/360
