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According to musl libc author:

"C99 requires the FE_ macros to be defined if and only if the exception they
correspond to is supported"

So we define these macros to 0 if they are not supported. Support for these FPU
exceptions is not necessary for correct functionality, but makes some tests less
effective.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55771

Signed-off-by: Uli Schlachter <psychon@znc.in>