run circle CI builds w/ additional CXX flags

[jadamcrain]

To avoid warnings from piling up during development, let's get our CI builds using these options:

-DCMAKE_CXX_FLAGS="-Wall -Werror"

[emgre]

It's done, but we exclude -Wformat-truncation, because the warnings are not important.

Do we want to add -Wextra ?

[jadamcrain]

It depends if there are warnings that are actually useful

[emgre]

-Wextra adds a couple of interesting warnings, most notably sign-compare and type-limits. But it also adds the very noisy and irrelevant unused-parameter that I manually disabled.

Added in d52c910 and 908fcf0.