Remove checks for deprecated _REENTRANT macro

According to feature_test_macros, this macro is obsolete and "glibc
has been thread-safe by default for many years." Other platforms
don't use it at all, for example FreeBSD.

Some parts of ROOT used it to determine if we are compiling with
-pthread, but this is essentially unused these days because the
CMake build always enables threading support and there exists no
option to turn it off.

lib/Interpreter/CIFactory.cpp

@@ -414,7 +414,7 @@ namespace {
     Opts.MathErrno = 0;
 #endif
 
-#ifdef _REENTRANT
+#ifndef _WIN32
     Opts.POSIXThreads = 1;
 #endif
 #ifdef __FAST_MATH__