detection of FILE definition under Cygwin

[Vincent Lefevre]

It seems that FILE definition is not detected under Cygwin. Sorry
for the lack of information, but in case someone can test or give
more information...

For the background, see

  https://stackoverflow.com/questions/49880944/cygwins-gcc-compiler-warning-from-mpfr-library

and my answer

  https://stackoverflow.com/a/49896200/3782797

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Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)