Last-gasp attempt to save libperl.so configure probe.

I notice that plperl's makefile adds the -I for $perl_archlibexp/CORE
at the end of CPPFLAGS not the beginning.  It seems somewhat unlikely
that the include search order has anything to do with why buildfarm
member okapi is failing, but I'm about out of other ideas.

configure

@@ -29127,7 +29127,7 @@ fi
 # check for <perl.h>
 if test "$with_perl" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="-I$perl_archlibexp/CORE $CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS -I$perl_archlibexp/CORE"
   { $as_echo "$as_me:$LINENO: checking for perl.h" >&5
 $as_echo_n "checking for perl.h... " >&6; }
 if test "${ac_cv_header_perl_h+set}" = set; then

configure.in

@@ -1795,7 +1795,7 @@ fi
 # check for <perl.h>
 if test "$with_perl" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="-I$perl_archlibexp/CORE $CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS -I$perl_archlibexp/CORE"
   AC_CHECK_HEADER(perl.h, [], [AC_MSG_ERROR([header file <perl.h> is required for Perl])],
                   [#include <EXTERN.h>])
   # While we're at it, check that we can link to libperl.