Stop linking with -lnsl on Linux

As far as I know on Linux libnsl is used only for NIS/NIS+ support.
It is highly unlikey that Rspamd users use NIS in 2018.

Keep linking with libnsl on Solaris - there it is likely needed even if
NIS is not used.

CMakeLists.txt

@@ -564,7 +564,6 @@ IF(CMAKE_SYSTEM_NAME STREQUAL "Linux")
 	LIST(APPEND CMAKE_REQUIRED_LIBRARIES rt)
 	LIST(APPEND CMAKE_REQUIRED_LIBRARIES dl)
 	LIST(APPEND CMAKE_REQUIRED_LIBRARIES resolv)
-	LIST(APPEND CMAKE_REQUIRED_LIBRARIES nsl)
 	MESSAGE(STATUS "Configuring for Linux")
 	IF(EXISTS "/etc/debian_version")
 		SET(LINUX_START_SCRIPT "rspamd_debian.in")