Fix configure's incorrect version tests for flex and perl.

awk's equality-comparison operator is "==" not "=".  We got this right
in many places, but not in configure's checks for supported version
numbers of flex and perl.  It hadn't been noticed because unsupported
versions are so old as to be basically extinct in the wild, and because
the only consequence is whether or not a WARNING flies by during
configure.

Daniel Gustafsson noted the problem with respect to the test for flex,
I found the other by reviewing other awk calls.

config/perl.m4

@@ -13,7 +13,7 @@ if test "$PERL"; then
   pgac_perl_version=`$PERL -v 2>/dev/null | sed -n ['s/This is perl.*v[a-z ]*\([0-9]\.[0-9][0-9.]*\).*$/\1/p']`
   AC_MSG_NOTICE([using perl $pgac_perl_version])
   if echo "$pgac_perl_version" | sed ['s/[.a-z_]/ /g'] | \
-    $AWK '{ if ([$]1 = 5 && [$]2 >= 8) exit 1; else exit 0;}'
+    $AWK '{ if ([$]1 == 5 && [$]2 >= 8) exit 1; else exit 0;}'
   then
     AC_MSG_WARN([
 *** The installed version of Perl, $PERL, is too old to use with PostgreSQL.

config/programs.m4

@@ -77,7 +77,7 @@ else
         echo '%%'  > conftest.l
         if $pgac_candidate -t conftest.l 2>/dev/null | grep FLEX_SCANNER >/dev/null 2>&1; then
           pgac_flex_version=`$pgac_candidate --version 2>/dev/null`
-          if echo "$pgac_flex_version" | sed ['s/[.a-z]/ /g'] | $AWK '{ if ([$]1 = 2 && ([$]2 > 5 || ([$]2 = 5 && [$]3 >= 31))) exit 0; else exit 1;}'
+          if echo "$pgac_flex_version" | sed ['s/[.a-z]/ /g'] | $AWK '{ if ([$]1 == 2 && ([$]2 > 5 || ([$]2 == 5 && [$]3 >= 31))) exit 0; else exit 1;}'
           then
             pgac_cv_path_flex=$pgac_candidate
             break 2

configure

@@ -7358,7 +7358,7 @@ else
         echo '%%'  > conftest.l
         if $pgac_candidate -t conftest.l 2>/dev/null | grep FLEX_SCANNER >/dev/null 2>&1; then
           pgac_flex_version=`$pgac_candidate --version 2>/dev/null`
-          if echo "$pgac_flex_version" | sed 's/[.a-z]/ /g' | $AWK '{ if ($1 = 2 && ($2 > 5 || ($2 = 5 && $3 >= 31))) exit 0; else exit 1;}'
+          if echo "$pgac_flex_version" | sed 's/[.a-z]/ /g' | $AWK '{ if ($1 == 2 && ($2 > 5 || ($2 == 5 && $3 >= 31))) exit 0; else exit 1;}'
           then
             pgac_cv_path_flex=$pgac_candidate
             break 2
@@ -7456,7 +7456,7 @@ if test "$PERL"; then
   { $as_echo "$as_me:${as_lineno-$LINENO}: using perl $pgac_perl_version" >&5
 $as_echo "$as_me: using perl $pgac_perl_version" >&6;}
   if echo "$pgac_perl_version" | sed 's/[.a-z_]/ /g' | \
-    $AWK '{ if ($1 = 5 && $2 >= 8) exit 1; else exit 0;}'
+    $AWK '{ if ($1 == 5 && $2 >= 8) exit 1; else exit 0;}'
   then
     { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING:
 *** The installed version of Perl, $PERL, is too old to use with PostgreSQL.