[native] Hacky test for __GLIBC__ to fix compile error on Alpine.

This appears to work; I tested it inside the Alpine chroot.

native/libc.c

@@ -59,9 +59,15 @@ func_fnmatch(PyObject *self, PyObject *args) {
     return NULL;
   }
 
-#ifdef _GNU_SOURCE
+  // NOTE: Testing for __GLIBC__ is the version detection anti-pattern.  We
+  // should really use feature detection in our configure script.  But I plan
+  // to get rid of the dependency on FNM_EXTMATCH because it doesn't work on
+  // musl libc (or OS X).  Instead we should compile extended globs to extended
+  // regex syntax.
+#ifdef __GLIBC__
   int flags = FNM_EXTMATCH;
 #else
+  debug("Warning: FNM_EXTMATCH is not defined");
   int flags = 0;
 #endif