Build inotify fsmonitor on BSDs

Check for existence of libinotify on some hardcoded paths (in addition
to the default include and library paths and any paths added by CFLAGS
or LDFLAGS) and if found then build inotify fsmonitor.

INSTALL.md

@@ -34,6 +34,10 @@ Please refer to instructions provided by the repository.
 
 - lablgtk3 and its prerequisites (ocamlfind, GTK 3 and its dependencies)
 
+##### Optional, on BSDs
+
+- libinotify (optional, for building unison-fsmonitor)
+
 ##### Optional, for building the user manual
 
 - LaTeX

src/Makefile.OCaml

@@ -324,6 +324,20 @@ ifeq ($(OSARCH),Linux)
   FSMDIR = fsmonitor/inotify
 endif
 
+ifneq ($(findstring $(OSARCH),FreeBSD OpenBSD NetBSD DragonFly),)
+  LIBINOTIFY_LIB:=-cclib '$(shell pkg-config --libs libinotify 2> /dev/null || printf ' -linotify')'
+  LIBINOTIFY_INC:=-ccopt '$(shell pkg-config --cflags libinotify 2> /dev/null)'
+  FOUND_LIBINOTIFY := $(shell { printf '' > inotifytest__.ml ;\
+    $(CAMLC) $(CAMLCFLAGS) $(CAMLLDFLAGS) $(LIBINOTIFY_LIB) \
+      -o inotifytest__ inotifytest__.ml > /dev/null 2>&1 && printf true ; } ;\
+    rm -f inotifytest__.ml inotifytest__.cm[oix] inotifytest__.o inotifytest__ > /dev/null 2>&1)
+  ifeq ($(FOUND_LIBINOTIFY), true)
+    FSMDIR = fsmonitor/inotify
+    $(NAME_FSM)$(EXEC_EXT): CAMLCFLAGS += $(LIBINOTIFY_INC)
+    $(NAME_FSM)$(EXEC_EXT): CLIBS += $(LIBINOTIFY_LIB)
+  endif
+endif
+
 ifeq ($(OSARCH),SunOS)
   FSMDIR = fsmonitor/solaris
 endif