Move the platform-dependent logic that sets CFLAGS_SL from
src/makefiles/Makefile.foo to src/template/foo, so that the value
is determined at configure time and thus is available while running
configure's tests.

On a couple of platforms this might save a few microseconds of build
time by eliminating a test that make otherwise has to do over and over.
Otherwise it's pretty much a wash for build purposes; in particular,
this makes no difference to anyone who might be overriding CFLAGS_SL
via a make option.

This patch in itself does nothing with the value and thus should not
change any behavior, though you'll probably have to re-run configure
to get a correctly updated Makefile.global.  We'll use the new
configure variable in a follow-on patch.

Per gripe from Kyotaro Horiguchi.  Back-patch to all supported branches,
because the follow-on patch is a portability bug fix.

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