Utility: fix build on FreeBSD and other BSD variants.

The _POSIX_C_SOURCE #if was a hopeful attempt, but apparently it didn't
work at all, due to _POSIX_C_SOURCE not being a thing at all unless
explicitly defined by the user. So instead testing for the BSD macros
directly.

Co-authored-by: Yuri Victorovich <yuri@FreeBSD.org>

doc/corrade-changelog.dox

@@ -303,7 +303,8 @@ namespace Corrade {
     you include @ref Corrade/Utility/DebugStlStringView.h
 -   Improved error handling in @ref Utility::Path APIs where all APIs now
     consistently print an error code and message coming from the system on
-    failure instead of a generic message or nothing at all
+    failure instead of a generic message or nothing at all. See also
+    [mosra/corrade#164](https://github.com/mosra/corrade/issues/164).
 -   @ref Utility::Path::isDirectory() now follows symlinks on Unix platforms
 -   @ref Utility::Path::ListFlag::SkipFiles and
     @ref Utility::Path::ListFlag::SkipDirectories passed to

src/Corrade/Utility/Implementation/ErrorString.cpp

@@ -60,7 +60,9 @@ void printErrnoErrorString(Debug& debug, const int error) {
        idea. The POSIX variant returns int(0) on success, while the GNU variant
        may return a pointer to a statically allocated string instead of filling
        the buffer. Sigh. */
-    #if ((_POSIX_C_SOURCE >= 200112L) && !_GNU_SOURCE) || defined(CORRADE_TARGET_EMSCRIPTEN) || defined(CORRADE_TARGET_APPLE)
+    /** @todo might want to add CORRADE_TARGET_BSD covering all three variants,
+        there's no overarching BSD define present on all BSDs :( */
+    #if ((_POSIX_C_SOURCE >= 200112L) && !_GNU_SOURCE) || defined(CORRADE_TARGET_EMSCRIPTEN) || defined(CORRADE_TARGET_APPLE) || defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
     char string[256];
     CORRADE_INTERNAL_ASSERT_OUTPUT(strerror_r(error, string, Containers::arraySize(string)) == 0);
     #else