utils: Fix check for fmemopen() fallback implementation

src/libstrongswan/utils/utils.c

@@ -515,7 +515,7 @@ _cas_impl(ptr, void*)
 #endif /* HAVE_GCC_ATOMIC_OPERATIONS */
 
 
-#if HAVE_FMEMOPEN == fallback
+#ifdef HAVE_FMEMOPEN_FALLBACK
 
 static int fmemread(chunk_t *cookie, char *buf, int size)
 {

src/libstrongswan/utils/utils.h

@@ -767,7 +767,8 @@ bool cas_ptr(void **ptr, void *oldval, void *newval);
 
 #ifndef HAVE_FMEMOPEN
 # ifdef HAVE_FUNOPEN
-#  define HAVE_FMEMOPEN fallback
+#  define HAVE_FMEMOPEN
+#  define HAVE_FMEMOPEN_FALLBACK
 /**
  * fmemopen(3) fallback using BSD funopen.
  *