FindOpenSSL: only try to parse opensslv.h if it exists

Modules/FindOpenSSL.cmake

@@ -218,7 +218,7 @@ ENDIF(WIN32 AND NOT CYGWIN)
 if (OPENSSL_INCLUDE_DIR)
   if (_OPENSSL_VERSION)
     set(OPENSSL_VERSION "${_OPENSSL_VERSION}")
-  else (_OPENSSL_VERSION)
+  elseif(OPENSSL_INCLUDE_DIR AND EXISTS "${OPENSSL_INCLUDE_DIR}/openssl/opensslv.h")
     file(STRINGS "${OPENSSL_INCLUDE_DIR}/openssl/opensslv.h" openssl_version_str
          REGEX "^#define[\t ]+OPENSSL_VERSION_NUMBER[\t ]+0x[0-9][0-9][0-9][0-9][0-9][0-9].*")