You cannot escape single quotes as the command itself is surrounded by single quotes, but you could use an octal escape code \047 to represent ' in POSIX awk. Additionally, you could use a hexadecimal escape code \x27 in GNU awk (gawk).
From The GNU Awk User’s Guide, 3.2 Escape Sequences:
\nnn
The octal value nnn, where nnn stands for 1 to 3 digits between ‘0’
and ‘7’. For example, the code for the ASCII ESC (escape) character is
\033.
\xhh…
The hexadecimal value hh, where hh stands for a sequence of
hexadecimal digits (‘0’–‘9’, and either ‘A’–‘F’ or ‘a’–‘f’). A maximum
of two digits are allowed after the \x. Any further hexadecimal
digits are treated as simple letters or numbers. (c.e.) (The \x
escape sequence is not allowed in POSIX awk.)
awk 'NR==90 { print "define(\`RELAY_MAILER_ARGS\047, \`TCP $h 2525\047)dnl"}1' example.com.mc
gawk 'NR==90 { print "define(\`RELAY_MAILER_ARGS\x27, \`TCP $h 2525\x27)dnl"}1' example.com.mc
If you would like to also use \140 (or \x60) to represent the backtick:
awk 'NR==90 { print "define(\140RELAY_MAILER_ARGS\047, \140TCP $h 2525\047)dnl"}1' example.com.mc
gawk 'NR==90 { print "define(\x60RELAY_MAILER_ARGS\x27, \x60TCP $h 2525\x27)dnl"}1' example.com.mc
