autoconf: Change quotes in description.

The unbalanced single-quotes cause many editor syntax highlighters
to interpret the rest of the file as a string literal, making it easier
to make syntax mistakes in absence of proper highlighting.

configure.ac

@@ -42,7 +42,7 @@ esac
 
 AC_MSG_RESULT($system)
 AC_SUBST(system)
-AC_DEFINE_UNQUOTED(SYSTEM, ["$system"], [platform identifier (`cpu-os')])
+AC_DEFINE_UNQUOTED(SYSTEM, ["$system"], [platform identifier ('cpu-os')])
 
 
 # State should be stored in /nix/var, unless the user overrides it explicitly.