The posix4 library has been merged into the libc library

Backported from commit 07e4dea.
Reference issue: libsdl-org#12588.

(cherry picked from commit d63b337)

Author: Dragon-Baroque

cmake/sdlchecks.cmake

@@ -966,7 +966,7 @@ macro(CheckPTHREAD)
       if(CMAKE_C_COMPILER_ID MATCHES "SunPro")
         set(PTHREAD_LDFLAGS "-mt -lpthread")
       else()
-        set(PTHREAD_LDFLAGS "-pthread -lposix4")
+        set(PTHREAD_LDFLAGS "-pthread")
       endif()
     elseif(SYSV5)
       set(PTHREAD_CFLAGS "-D_REENTRANT -Kthread")