[Rev 02] RFR: 8241476: Linux build warning issued on updated compilers (Ubuntu 18.04.4 / 20.04)

[Kevin Rushforth]

On Tue, 31 Mar 2020 20:45:44 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

>> 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.
>
> Please, let me know if this is the desired way to do it. If not, I will rework it. Thanks.

This is roughly what I had in mind. I want to test it, and also take a look at the WebKit and media builds as well.
WebKit uses the flags from `linux.gradle`, and is mostly C++, so I think it needs no changes. Media does not use any
flags from `linux.gradle` -- only the jfxmedia project has C++ files on Linux. I guess it should be fine, too, but I'll
see.

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