hamt: Fix compilation error on NetBSD 7

[Bruno Haible]

On NetBSD 7, I get a compilation error in hamt.h, at the token '_Atomic'.

On this platform, GCC is 4.8.x, __STDC_VERSION__ is 201112L, and
__STD_NO_ATOMICS__ is not defined. In these conditions, hamt.h defines
GL_HAMT_THREAD_SAFE to 1. It should define it to 0 instead.


2021-12-24  Bruno Haible  <bruno@clisp.org>

        hamt: Fix compilation error on NetBSD 7.
        * lib/hamt.h (GL_HAMT_THREAD_SAFE): Define to 0 if the compiler is a
        GCC version < 4.9.

diff --git a/lib/hamt.h b/lib/hamt.h
index 5d553fc7d..440f34acc 100644
--- a/lib/hamt.h
+++ b/lib/hamt.h
@@ -56,13 +56,16 @@ _GL_INLINE_HEADER_BEGIN
 /* The GL_HAMT_THREAD_SAFE flag is set if the implementation of hamts
    is thread-safe as long as two threads do not simultaneously access
    the same hamt.  This is non-trivial as different hamts may share
-   some structure.  */
+   some structure.
+   We can define it only when the compiler supports _Atomic.  For GCC,
+   it is supported starting with GCC 4.9.  */
 
-#if (__STDC_VERSION__ < 201112 || defined __STD_NO_ATOMICS__) \
-  && __GNUC__ + (__GNUC_MINOR >= 9) <= 4
-# define GL_HAMT_THREAD_SAFE 0
-#else
+#if (__GNUC__ + (__GNUC_MINOR >= 9) > 4) \
+    && __STDC_VERSION__ >= 201112L && !defined __STD_NO_ATOMICS__ \
+    && !defined __cplusplus
 # define GL_HAMT_THREAD_SAFE 1
+#else
+# define GL_HAMT_THREAD_SAFE 0
 #endif
 
 #include <stdbool.h>