fix freebsd build

cmake/unix.cmake

@@ -2,16 +2,21 @@ add_definitions(-DUNIX)
 add_definitions(-DPOSIX)
 
 if(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+  set(FS_LIB stdc++fs)
   get_filename_component(LIBTUNTAP_IMPL ${TT_ROOT}/tuntap-unix-linux.c ABSOLUTE)
 elseif(${CMAKE_SYSTEM_NAME} MATCHES "Android")
+
+  set(FS_LIB stdc++fs)
   get_filename_component(LIBTUNTAP_IMPL ${TT_ROOT}/tuntap-unix-linux.c ABSOLUTE)
 elseif (${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD")
   set(LIBTUNTAP_IMPL ${TT_ROOT}/tuntap-unix-openbsd.c ${TT_ROOT}/tuntap-unix-bsd.c)
 elseif (${CMAKE_SYSTEM_NAME} MATCHES "NetBSD")
   set(LIBTUNTAP_IMPL ${TT_ROOT}/tuntap-unix-netbsd.c ${TT_ROOT}/tuntap-unix-bsd.c)
 elseif (${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD" OR ${CMAKE_SYSTEM_NAME} MATCHES "DragonFly")
+  set(FS_LIB c++experimental)
   set(LIBTUNTAP_IMPL ${TT_ROOT}/tuntap-unix-freebsd.c ${TT_ROOT}/tuntap-unix-bsd.c)
 elseif (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+  set(FS_LIB stdc++fs)
   set(LIBTUNTAP_IMPL ${TT_ROOT}/tuntap-unix-darwin.c ${TT_ROOT}/tuntap-unix-bsd.c)
 elseif (${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
   set(LIBTUNTAP_IMPL ${TT_ROOT}/tuntap-unix-sunos.c)
@@ -19,7 +24,6 @@ else()
   message(FATAL_ERROR "Your operating system is not supported yet")
 endif()
 
-set(FS_LIB stdc++fs)
 
 set(EXE_LIBS ${STATIC_LIB} libutp)