build: allow user to override CFLAGS

Now something like this works as expected:
 cmake -DCMAKE_C_FLAGS="-O3 -Wno-error"

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
build: remove -Werror from default CFLAGS

It usually breaks on each untested compiler version. That's annoying for
the user. On our own devel machines we can always explicitly set it and
we still set it per default in debug mode.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
build: set CXXFLAGS also like CFLAGS

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
travis-ci: compile again with -Werror

-Werror was removed lately from default CFLAGS. In CXXFLAGS it was
never used until now.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>