Check for build environment instead of just _GNU_SOURCE

Fixes the build on OpenBSD and NetBSD. It is too easy for _GNU_SOURCE
to be defined even on non-Linux systems. Found via py-zstandard with
the embedded copy of zstandard and Python defines _GNU_SOURCE.

Also simplify the Linux checking, there is no need to check the rest
of the symbol names.

Author: brad0

lib/dictBuilder/cover.c

@@ -77,7 +77,7 @@
 #ifndef ZDICT_QSORT
 # if defined(__APPLE__)
 #   define ZDICT_QSORT ZDICT_QSORT_APPLE /* uses qsort_r() with a different order for parameters */
-# elif defined(_GNU_SOURCE)
+# elif defined(__linux__) || defined(__CYGWIN__) || defined(__MSYS__)
 #   define ZDICT_QSORT ZDICT_QSORT_GNU /* uses qsort_r() */
 # elif defined(_WIN32) && defined(_MSC_VER)
 #   define ZDICT_QSORT ZDICT_QSORT_MSVC /* uses qsort_s() with a different order for parameters */