shell - What is the difference ` and ' in the sendmail configuration file? -

i reading article :
http://linuxconfig.org/configuring-gmail-as-sendmail-email-relay.

in middle of article;

put bellow lines sendmail.mc configuration file right above first "mailer" definition line:  define(`smart_host',`[smtp.gmail.com]')dnl define(`relay_mailer_args', `tcp $h 587')dnl define(`esmtp_mailer_args', `tcp $h 587')dnl define(`confauth_options', `a p')dnl trust_auth_mech(`external digest-md5 cram-md5 login plain')dnl define(`confauth_mechanisms', `external gssapi digest-md5 cram-md5 login plain')dnl feature(`authinfo',`hash -o /etc/mail/authinfo/gmail-auth.db')dnl 

i wondering there difference ` , ' here?
why don't write

define('smart_host','[smtp.gmail.com]')dnl define('relay_mailer_args', 'tcp $h 587')dnl define('esmtp_mailer_args', 'tcp $h 587')dnl define('confauth_options', 'a p')dnl trust_auth_mech('external digest-md5 cram-md5 login plain')dnl define('confauth_mechanisms', 'external gssapi digest-md5 cram-md5 login plain')dnl feature('authinfo','hash -o /etc/mail/authinfo/gmail-auth.db')dnl 

in opinion changed file more elegant , contain symmetry in it,two ' enclose string ,who can express that?

and truth changed lines can run ,i tested it.

sendmail configuration processed m4.

read gnu m4 documentation, in particular section on quoting input