Description:
------------
(php-7.4.7/ext/session/mod_mm.c) /    469: return value type does not match the function type
          return nrdels;

This points to PS_GC_FUNC(mm).

PS_GC_FUNC is:
#define PS_GC_FUNC(x)           zend_long ps_gc_##x(PS_GC_ARGS)  <-- Note zend_long return type

And PS_GC_ARGS is:
#define PS_GC_ARGS       void **mod_data, zend_long maxlifetime, zend_long *nrdels  <-- Note zend_long* pointer type

PS_GC_FUNC(mm) returns nrdels which is a zend_long pointer but its return type is just a zend_long value.

The returns statement should be:
return *nrdels;