configure*: Fix wrong quote character in AC_MSG_ERROR

Square brackets are used as quotes in autoconf.

From Section 8.1.1, Active Characters of the Autoconf manual[1]:

> To fully understand where proper quotation is important, you first
> need to know what the special characters are in Autoconf: ‘#’
> introduces a comment inside which no macro expansion is performed, ‘,’
> separates arguments, ‘[’ and ‘]’ are the quotes themselves, ‘(’ and
> ‘)’ (which M4 tries to match by pairs), and finally ‘$’ inside a macro
> definition.

[1] https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.70/autoconf.html#Active-Characters

configure

@@ -3643,7 +3643,7 @@ fi
 
 
  if test "x$HAVE_GAWK" != "xyes"; then :
- as_fn_error $? "\"*** gawk not found ***\"" "$LINENO" 5
+ as_fn_error $? "*** gawk not found ***" "$LINENO" 5
 fi
 
 fi

configure.ac

@@ -120,7 +120,7 @@ AS_IF([test "x$enable_man" != "xno"], [
  HAVE_MAN="-DHAVE_MAN"
  AC_SUBST(HAVE_MAN)
  AC_CHECK_PROG([HAVE_GAWK], [gawk], [yes], [no])
- AS_IF([test "x$HAVE_GAWK" != "xyes"], [AC_MSG_ERROR("*** gawk not found ***")])
+ AS_IF([test "x$HAVE_GAWK" != "xyes"], [AC_MSG_ERROR([*** gawk not found ***])])
 ])
 
 HAVE_FIRETUNNEL=""