[Rev 01] RFR: 8241476: Linux build warning issued on updated compilers (Ubuntu 18.04.4 / 20.04)

[Kevin Rushforth]

On Thu, 26 Mar 2020 08:33:29 GMT, Johan Vos <jvos at openjdk.org> wrote:

>> Like this?
>> 
>> I think all C builds are covered. Gstreamer seems to have a Makefile with the flags. Not sure about libxml and libxlst
>> inside javafx.web.
>
> I think it's better to split the flags between CFLAGS and CPPFLAGS (as is done in OpenJDK). The
> `-Werror=implicit-function-declaration` is extremely useful for C files, and we don't want to risk that this somehow
> got lost. If we use CFLAGS and CPPFLAGS, we can pass the latter as CXXFLAGS to cmake, so that should be fairly easy.

I took a look yesterday and came to the same conclusion that what we really want are separate C and C++ flags. For now,
the only difference would be the presence or absence of `-Werror=implicit-function-declaration`, but it would allow for
other differences in the future if needed.

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PR: https://git.openjdk.java.net/jfx/pull/150