Put back OPENSSL_SYS_MSDOS definition to revive DJGPP built.

PR: 1247

e_os2.h

@@ -84,6 +84,13 @@ extern "C" {
 
 /* ---------------------- Microsoft operating systems ---------------------- */
 
+/* Note that MSDOS actually denotes 32-bit environments running on top of
+   MS-DOS, such as DJGPP one. */
+#if defined(OPENSSL_SYSNAME_MSDOS)
+# undef OPENSSL_SYS_UNIX
+# define OPENSSL_SYS_MSDOS
+#endif
+
 /* For 32 bit environment, there seems to be the CygWin environment and then
    all the others that try to do the same thing Microsoft does... */
 #if defined(OPENSSL_SYSNAME_UWIN)