GCRYCTL_PRINT_CONFIG can be used to print
configuration options, it is sometimes necessary to check them in a
program. This can be accomplished by using this function:
This function returns a malloced string with colon delimited configure
options. With a value of 0 for mode this string resembles the
GCRYCTL_PRINT_CONFIG. However, if what is not
NULL, only the line where the first field (e.g. "cpu-arch") matches
what is returned.
Other values than 0 for mode are not defined. The caller shall
free the string using
gcry_free. On error NULL is returned and
ERRNO is set; if a value for WHAT is unknow ERRNO will be set to 0.