Use CFLAGS_SL while probing linkability of libperl.

On recent Red Hat platforms (at least RHEL 8 and Fedora 30, maybe older),
configure's probe for libperl failed if the user forces CFLAGS to be -O0.
This is because some code in perl's inline.h fails to be optimized away
at -O0, and said code doesn't work if compiled without -fPIC.

To fix, add CFLAGS_SL to the compile flags used during the libperl probe.
This is a better simulation of the way that plperl is built, anyway,
so it might forestall other issues in future.

Per gripe from Kyotaro Horiguchi.  Back-patch to all supported branches,
since people might want to build older branches on these platforms.

Discussion: https://postgr.es/m/20191010.144533.263180400.horikyota.ntt@gmail.com

configure

@@ -18490,6 +18490,10 @@ fi
   # On most platforms, if perl.h is there then libperl.so will be too, but at
   # this writing Debian packages them separately.  There is no known reason to
   # waste cycles on separate probes for the Tcl or Python libraries, though.
+  # On some Red Hat platforms, the link attempt can fail if we don't use
+  # CFLAGS_SL while building the test program.
+  ac_save_CFLAGS=$CFLAGS
+  CFLAGS="$CFLAGS $CFLAGS_SL"
   pgac_save_LIBS=$LIBS
   LIBS="$perl_embed_ldflags"
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking for libperl" >&5
@@ -18519,6 +18523,7 @@ fi
 rm -f core conftest.err conftest.$ac_objext \
     conftest$ac_exeext conftest.$ac_ext
   LIBS=$pgac_save_LIBS
+  CFLAGS=$ac_save_CFLAGS
   CPPFLAGS=$ac_save_CPPFLAGS
 fi
 

configure.in

@@ -2253,6 +2253,10 @@ if test "$with_perl" = yes; then
   # On most platforms, if perl.h is there then libperl.so will be too, but at
   # this writing Debian packages them separately.  There is no known reason to
   # waste cycles on separate probes for the Tcl or Python libraries, though.
+  # On some Red Hat platforms, the link attempt can fail if we don't use
+  # CFLAGS_SL while building the test program.
+  ac_save_CFLAGS=$CFLAGS
+  CFLAGS="$CFLAGS $CFLAGS_SL"
   pgac_save_LIBS=$LIBS
   LIBS="$perl_embed_ldflags"
   AC_MSG_CHECKING([for libperl])
@@ -2264,6 +2268,7 @@ if test "$with_perl" = yes; then
     [AC_MSG_RESULT(no)
      AC_MSG_ERROR([libperl library is required for Perl])])
   LIBS=$pgac_save_LIBS
+  CFLAGS=$ac_save_CFLAGS
   CPPFLAGS=$ac_save_CPPFLAGS
 fi