Replace FSMonitor v2 with v4

Makefile

@@ -473,8 +473,13 @@ all::
 #
 # If your platform supports a built-in fsmonitor backend, set
 # FSMONITOR_DAEMON_BACKEND to the "<name>" of the corresponding
-# `compat/fsmonitor/fsmonitor-fs-listen-<name>.c` that implements the
-# `fsmonitor_fs_listen__*()` routines.
+# `compat/fsmonitor/fsm-listen-<name>.c` that implements the
+# `fsm_listen__*()` routines.
+#
+# If your platform has os-specific ways to tell if a repo is incompatible with
+# fsmonitor (whether the hook or ipc daemon version), set FSMONITOR_OS_SETTINGS
+# to the "<name>" of the corresponding `compat/fsmonitor/fsm-settings-<name>.c`
+# that implements the `fsm_os_settings__*()` routines.
 #
 # Define DEVELOPER to enable more compiler warnings. Compiler version
 # and family are auto detected, but could be overridden by defining
@@ -909,6 +914,7 @@ LIB_OBJS += fmt-merge-msg.o
 LIB_OBJS += fsck.o
 LIB_OBJS += fsmonitor.o
 LIB_OBJS += fsmonitor-ipc.o
+LIB_OBJS += fsmonitor-settings.o
 LIB_OBJS += gettext.o
 LIB_OBJS += gpg-interface.o
 LIB_OBJS += graph.o
@@ -1946,7 +1952,12 @@ endif
 
 ifdef FSMONITOR_DAEMON_BACKEND
 	COMPAT_CFLAGS += -DHAVE_FSMONITOR_DAEMON_BACKEND
-	COMPAT_OBJS += compat/fsmonitor/fsmonitor-fs-listen-$(FSMONITOR_DAEMON_BACKEND).o
+	COMPAT_OBJS += compat/fsmonitor/fsm-listen-$(FSMONITOR_DAEMON_BACKEND).o
+endif
+
+ifdef FSMONITOR_OS_SETTINGS
+	COMPAT_CFLAGS += -DHAVE_FSMONITOR_OS_SETTINGS
+	COMPAT_OBJS += compat/fsmonitor/fsm-settings-$(FSMONITOR_OS_SETTINGS).o
 endif
 
 ifeq ($(TCLTK_PATH),)
@@ -2833,6 +2844,9 @@ GIT-BUILD-OPTIONS: FORCE
 ifdef FSMONITOR_DAEMON_BACKEND
 	@echo FSMONITOR_DAEMON_BACKEND=\''$(subst ','\'',$(subst ','\'',$(FSMONITOR_DAEMON_BACKEND)))'\' >>$@+
 endif
+ifdef FSMONITOR_OS_SETTINGS
+	@echo FSMONITOR_OS_SETTINGS=\''$(subst ','\'',$(subst ','\'',$(FSMONITOR_OS_SETTINGS)))'\' >>$@+
+endif
 ifdef TEST_OUTPUT_DIRECTORY
 	@echo TEST_OUTPUT_DIRECTORY=\''$(subst ','\'',$(subst ','\'',$(TEST_OUTPUT_DIRECTORY)))'\' >>$@+
 endif