FindThreads: Restore hard-coded '-l' flag on library name

Revert commit de5f123d3a (FindThreads: Do not hard-code '-l' flag on
library name, 2019-09-23, v3.16.0-rc1~71^2).  Some clients depend
on the value of `CMAKE_THREAD_LIBS_INIT` to contain a valid flag.

Fixes: #20061, #20065

Modules/FindThreads.cmake

@@ -77,7 +77,7 @@ macro(_check_threads_lib LIBNAME FUNCNAME VARNAME)
   if(NOT Threads_FOUND)
      CHECK_LIBRARY_EXISTS(${LIBNAME} ${FUNCNAME} "" ${VARNAME})
      if(${VARNAME})
-       set(CMAKE_THREAD_LIBS_INIT "${LIBNAME}")
+       set(CMAKE_THREAD_LIBS_INIT "-l${LIBNAME}")
        set(CMAKE_HAVE_THREADS_LIBRARY 1)
        set(Threads_FOUND TRUE)
      endif()
@@ -88,7 +88,7 @@ endmacro()
 # Do NOT even think about using it outside of this file!
 macro(_check_pthreads_flag)
   if(NOT Threads_FOUND)
-    # If we did not find a thread library look for -pthread compiler option.
+    # If we did not found -lpthread, -lpthread, or -lthread, look for -pthread
     if(NOT DEFINED THREADS_HAVE_PTHREAD_ARG)
       message(STATUS "Check if compiler accepts -pthread")
       if(CMAKE_C_COMPILER_LOADED)
@@ -164,7 +164,7 @@ if(CMAKE_HAVE_PTHREAD_H)
       _check_threads_lib(pthreads pthread_create CMAKE_HAVE_PTHREADS_CREATE)
       _check_threads_lib(pthread  pthread_create CMAKE_HAVE_PTHREAD_CREATE)
       if(CMAKE_SYSTEM_NAME MATCHES "SunOS")
-          # On sun also check for thread library with thr_create
+          # On sun also check for -lthread
           _check_threads_lib(thread thr_create CMAKE_HAVE_THR_CREATE)
       endif()
     endif()
@@ -195,7 +195,7 @@ if(CMAKE_USE_PTHREADS_INIT)
     # are available.
     CHECK_LIBRARY_EXISTS(cma pthread_attr_create "" CMAKE_HAVE_HP_CMA)
     if(CMAKE_HAVE_HP_CMA)
-      set(CMAKE_THREAD_LIBS_INIT "cma")
+      set(CMAKE_THREAD_LIBS_INIT "-lcma")
       set(CMAKE_HP_PTHREADS_INIT 1)
       set(Threads_FOUND TRUE)
     endif()