[PATCH] build: revise visibility attributes usage in autotools and cmake

[Ozkan Sezer]

* cmake: set C_VISIBILITY_PRESET to hidden for non-windows only.
  windows handles exports with DLL_EXPORT defined, and old mingw
  toolchains emit a lot of warnings about visibility attributes
  not being supported in this configuration.
* autotools: revise visibility attributes checks by compiling a
  better test program with `-Werror` in CFLAGS, so that warnings
  will correctly indicate unsupported configurations.

Motivation for this patch was annoying warnings from my mingw
toolchains with gcc 7.5 and gcc 4.5

P.S.: I don't know how to do this in meson, and it's probably not
worth it.

--
O.S.

0001-build-revise-visibility-attributes-usage-in-autotool.patch
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