Add CXXFLAGS=-Wp,-D_GLIBCXX_ASSERTIONS to the gcc-9 build in Travis CI

[esabol]

As discussed in issue #272, ensure C++ STL is used properly, according to _GLIBCXX_ASSERTIONS. This only adds it to the gcc-9 build. Let me know if you think it should be added to other gcc/g++ builds.

Reference:
https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_macros.html

[esabol]

Do not merge this. I must have done something wrong. The gcc/g++-9 build should have failed with _GLIBCXX_ASSERTIONS.

[SpamapS]

Ed you can reopen if/when it's safe. Just don't want us to accidentally merge something weird. :)

[esabol]

Ed you can reopen if/when it's safe.

I don't see a button to reopen it?

I figured it out and have tested it here:
https://travis-ci.org/esabol/gearmand/jobs/653268979

This build also crashes t/unittest, which is interesting. @cheese didn't report seeing that on Fedora 31.

[esabol]

This build also crashes t/unittest, which is interesting. @cheese didn't report seeing that on Fedora 31.

Oh, that's the libtest/http.cc issue. Never mind. It's all good. I'll rebase this PR once PR #281 is merged, and the gcc-9/g++-9 build should work again.

[p-alik]

I don't see a button to reopen it?

You'll need at least contributor permissions to reopen an issue.
@SpamapS, could you grant it to @esabol?

[SpamapS]

Quoting Алексей Пастухов (2020-02-21 02:04:02)

I don't see a button to reopen it? You'll need at least contributor permissions to reopen an issue. ***@***.***, could you grant it to ***@***.***?

Invite sent for write access. Congrats Ed! ;)

[esabol]

This PR is blocked by PR #281, fwiw. (Edited. Sorry.)