file_watch: only use inotify on linux

Fixes build on *BSD platforms.

Fixes #262, Closes #261

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/meson.build

@@ -78,6 +78,14 @@ if get_option('unittest')
 	cflags += ['-DUNIT_TEST']
 endif
 
+host_system = host_machine.system()
+if host_system == 'linux'
+	cflags += ['-DHAS_INOTIFY']
+elif host_system == 'freebsd' or host_system == 'netbsd' or
+     host_system == 'dragonfly' or host_system == 'openbsd'
+	cflags += ['-DHAS_KQUEUE']
+endif
+
 subdir('backend')
 
 picom = executable('picom', srcs, c_args: cflags,