don't require libraries that aren't needed

krb5-config (used by FindGSSAPI) returns `-lkrb5 -lk5crypto -lcom_err`
but only libkrb5 is actually used by the plugin. The other two result
in unneeded dependencies unless they're tagged optional when linked
with --as-needed.

Some distributions use --as-needed automatically, which causes our
builds to differ from srpm builds, introducing failures in buildbot.

plugins/auth/CMakeLists.txt

@@ -99,6 +99,8 @@ IF(NOT WIN32)
   INCLUDE(${CC_SOURCE_DIR}/cmake/FindGSSAPI.cmake)
   IF(GSSAPI_FOUND)
     SET(GSSAPI_SOURCES ${AUTH_DIR}/auth_gssapi_client.c ${AUTH_DIR}/gssapi_client.c ${AUTH_DIR}/gssapi_errmsg.c)
+    INCLUDE(CheckLinkerFlag)
+    CHECK_LINKER_FLAG(C -Wl,--as-needed have_C__Wl___as_needed)
   ENDIF()
 ELSE()
   SET(GSSAPI_LIBS secur32)
@@ -115,6 +117,9 @@ IF(GSSAPI_SOURCES)
   IF(CMAKE_C_COMPILER_ID MATCHES "Clang")
     SET_SOURCE_FILES_PROPERTIES(${GSSAPI_SOURCES} PROPERTY COMPILE_FLAGS "-Wno-deprecated-declarations")
   ENDIF()
+  IF(have_C__Wl___as_needed)
+    SET_TARGET_PROPERTIES(auth_gssapi_client PROPERTIES LINK_FLAGS "-Wl,--as-needed")
+  ENDIF()
 ENDIF()
 
 # old_password plugin