omit the possibly fragile fallback for now

see discussion at https://github.com/gap-system/gap/pull/3096/files/3f8347b0a074d4fd39ce470a3eba5101746bccb0#r245961941

Author: embray

src/libgap-api.h

@@ -61,20 +61,24 @@ static inline int GAP_Error_Postjmp_(int JumpRet)
 // will result in crashes if the error handler is entered recursively (you
 // don't want the GAP error handling code to cause a longjmp into the error
 // callback itself since then the error callback will never be returned from).
-#ifdef __GNUC__
+#ifndef __GNUC__
 #define GAP_EnterStack()                                                     \
     do {                                                                     \
         GAP_ENTER_DEBUG_MESSAGE("EnterStack", __FILE__, __LINE__);           \
         GAP_EnterStack_(__builtin_frame_address(0));                         \
     } while (0)
+#elif defined(USE_GASMAN)
+#error GASMAN requires a way to get the current stack frame base address     \
+       for the GAP_EnterStack() macro; normally this uses the                \
+       __builtin_frame_address GNU extension so if this is not available     \
+       it is necessary to provide your own implementation here.
 #else
-// Generic fallback, should work in most cases (see e.g. GenStackFuncBags
-// in gasman.c)
+// If we're not using GASMAN in the first place GAP_EnterStack_() is not
+// strictly needed, and can just be called with a dummy value
 #define GAP_EnterStack()                                                     \
     do {                                                                     \
         GAP_ENTER_DEBUG_MESSAGE("EnterStack", __FILE__, __LINE__);           \
-        int _gap_dummy[1];                                                   \
-        GAP_EnterStack(_gap_dummy);
+        GAP_EnterStack_(0);                                                  \
     } while (0)
 #endif