Fix strdup macro redefinition

This fixes the following error when the CRT debug heap (crtdbg.h) is used:
e_os.h(476): warning C4005: 'strdup': macro redefinition
C:\Program Files (x86)\Windows Kits\10\Include\10.0.10586.0\ucrt\crtdbg.h(319): note: see previous definition of 'strdup'
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1137)

e_os.h

@@ -473,7 +473,9 @@ struct servent *PASCAL getservbyname(const char *, const char *);
 #   define open _open
 #   define fdopen _fdopen
 #   define close _close
-#   define strdup _strdup
+#   ifndef strdup
+#    define strdup _strdup
+#   endif
 #   define unlink _unlink
 #  endif
 # else