Avoid portability problem introduced in 0a42a2e9c.

Apparently not all shells handle nested quotes quite the same.
But we don't need the inner double quotes in this case, so just
drop them.

Discussion: https://postgr.es/m/50ecb345-4dc5-1f7d-64ca-7018195fcc8d@2ndquadrant.com

configure

@@ -18887,7 +18887,7 @@ _ACEOF
 
 # Supply a numeric version string for use by 3rd party add-ons
 # awk -F is a regex on some platforms, and not on others, so make "." a tab
-PG_VERSION_NUM="`echo "$PG_MAJORVERSION	$PG_MINORVERSION" |
+PG_VERSION_NUM="`echo $PG_MAJORVERSION $PG_MINORVERSION |
 $AWK '{printf "%d%04d", $1, $2}'`"
 
 cat >>confdefs.h <<_ACEOF

configure.in

@@ -2322,7 +2322,7 @@ AC_DEFINE_UNQUOTED(PG_VERSION_STR,
 
 # Supply a numeric version string for use by 3rd party add-ons
 # awk -F is a regex on some platforms, and not on others, so make "." a tab
-[PG_VERSION_NUM="`echo "$PG_MAJORVERSION	$PG_MINORVERSION" |
+[PG_VERSION_NUM="`echo $PG_MAJORVERSION $PG_MINORVERSION |
 $AWK '{printf "%d%04d", $1, $2}'`"]
 AC_DEFINE_UNQUOTED(PG_VERSION_NUM, $PG_VERSION_NUM, [PostgreSQL version as a number])
 AC_SUBST(PG_VERSION_NUM)