Fix compilation error with Mingw32

Fix compilation error on Mingw32 when `_TRUNCATE` is defined. Use
`_TRUNCATE` only if `__MINGW32__` not defined. Fix suggested by
Thomas Glanzmann and Nick Wilson on issue #355
diff --git a/ChangeLog b/ChangeLog
index ed7818e..80d4cf5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -62,6 +62,9 @@
    * Fix issue in RSA key generation program programs/x509/rsa_genkey
      where the failure of CTR DRBG initialization lead to freeing an
      RSA context without proper initialization beforehand.
+   * Fix compilation error on Mingw32 when `_TRUNCATE` is defined. Use `_TRUNCATE`
+     only if `__MINGW32__` not defined. Fix suggested by Thomas Glanzmann and
+     Nick Wilson on issue #355
 
 Changes
    * Extend cert_write example program by options to set the CRT version