Post-MDEV-30700: moving alloca() definitions from all *.h files to new header file

Included config file for proper compilation without <my_global.h>

include/my_alloca.h

@@ -16,6 +16,8 @@
 #ifndef MY_ALLOCA_INCLUDED
 #define MY_ALLOCA_INCLUDED
 
+#include <my_config.h>
+
 #ifdef _WIN32
 #include <malloc.h> /*for alloca*/
 /*