Make the disablement of optimizatiomn in x86's thunkProtect specific to

gcc 7.4.x.  Guille, Pablo, All, please take note of this style.  The point
being we want correctness, but we shouldn't penalise unnecessarily other
platfoms or build configurations that do function correctly.

platforms/Cross/plugins/IA32ABI/ia32abicc.c

@@ -168,8 +168,18 @@ getMostRecentCallbackContext() { return mostRecentCallbackContext; }
  * longs, followed by the thunkp and stackp passed on the stack.  The register
  * args would get copied into a struct on the stack. A pointer to the struct
  * is then passed as an element of the VMCallbackContext.
+ *
+ * N.B. On gcc 7.4.x thunkEntry fails if optimized, for as yet not fully
+ * diagnosed reasons.  See
+ *     https://github.com/OpenSmalltalk/opensmalltalk-vm/pull/353
+ * Hence the pragma below.
  */
-long __attribute__((optimize("O0")))
+long
+#ifdef __GNUC__
+#  if __GNUC__ == 7 && __GNUC_MINOR__ >= 4
+__attribute__((optimize("O0")))
+#  endif
+#endif
 thunkEntry(void *thunkp, sqIntptr_t *stackp)
 {
 	VMCallbackContext vmcc;