Fix O_RDONLY (_O_RDONLY) under mingw

O_RDONLY is defined only if [1]:

    !defined(NO_OLDNAMES) || defined(_POSIX)

  [1]: https://github.com/Alexpux/mingw-w64/blob/d0d7f784833bbb0b2d279310ddc6afb52fe47a46/mingw-w64-headers/crt/fcntl.h#L35

Author: azat

arc4random.c

@@ -89,6 +89,10 @@ struct arc4_stream {
 #define pid_t int
 #endif
 
+#ifndef O_RDONLY
+#define O_RDONLY _O_RDONLY
+#endif
+
 static int rs_initialized;
 static struct arc4_stream rs;
 static pid_t arc4_stir_pid;

evutil.c

@@ -109,6 +109,10 @@
 #define mode_t int
 #endif
 
+#ifndef O_RDONLY
+#define O_RDONLY _O_RDONLY
+#endif
+
 #ifdef EVENT__HAVE_AFUNIX_H
 int have_working_afunix_ = -1;
 #endif