fixup! mimalloc: offer a build-time option to enable it

To make `MIMALLOC_SHOW_STATS` work (setting this environment variable
will print out statistics about the allocations in the process just
before it terminates), we need to use mimalloc's "Fiber" based process
termination handling that was introduced in aa881733 (reorganize
primitives for process initialization; use special data segment on
Windows for thread termination by default on Windows now (issue git-for-windows#869),
2024-10-21): It requires UCRT which we do not use in Git for Windows'
GCC-based builds.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Makefile

@@ -2195,6 +2195,7 @@ ifdef USE_MIMALLOC
 $(MIMALLOC_OBJS): COMPAT_CFLAGS += -DBANNED_H
 
 $(MIMALLOC_OBJS): COMPAT_CFLAGS += \
+	-DMI_WIN_USE_FLS \
 	-Wno-attributes \
 	-Wno-unknown-pragmas \
 	-Wno-unused-function \