With the new release of the GCC 9 compiler the operation of -Wmissing-attributes warnings has also been extended to aliases: this causes a warnings if the alias has less attributes than its target.
This warnings does not actually indicate a problem in the mimalloc code and you could safely ignore it by adding "-Wno-missing-attributes" to the CFLAGS, however I was not going to use a "hack" to hide the warning when compiling alloc-override .c
So this patch solves the problem simply by using an attribute (present only in GCC> = 9) that copies the attributes of another function, since this attribute is exclusive only for GCC 9 (or later) I had to use a simple precompiler instruction to add the "copy" attribute only with GCC 9.
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