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

Now that Git is a bit stricter when compiling its own source, we need to
exempt mimalloc a bit more.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

Makefile

@@ -2201,7 +2201,8 @@ $(MIMALLOC_OBJS): COMPAT_CFLAGS += \
 	-Wno-pedantic \
 	-Wno-declaration-after-statement \
 	-Wno-old-style-definition \
-	-Wno-missing-prototypes
+	-Wno-missing-prototypes \
+	-Wno-implicit-function-declaration
 endif
 endif