Fix AC_CHECK_DECLS usage in openssl.m4.

Fix AC_CHECK_DECLS usage in openssl.m4.

See:

  https://www.gnu.org/software/autoconf/manual/autoconf-2.62/html_node/Generic-Declarations.html

"For each of the symbols (comma-separated list)"

When building with aggressive warning settings the current code results
in the following configure test code being generated:

  #ifndef OpenSSL_add_all_algorithms EVP_aes_256_cfb
  #ifdef __cplusplus
    (void) OpenSSL_add_all_algorithms EVP_aes_256_cfb;
  #else
    (void) OpenSSL_add_all_algorithms EVP_aes_256_cfb;
  #endif
  #endif

Which is obviously wrong and makes the configure check fail.