pgsql: Select CFLAGS_SL at configure time, not in platform-specific Mak

Select CFLAGS_SL at configure time, not in platform-specific Makefiles.

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

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e167b1ae37ad0dad985b843c6ba654c3619ced26

Modified Files
--------------
configure                      | 5 ++++-
configure.in                   | 5 ++++-
src/Makefile.global.in         | 1 +
src/makefiles/Makefile.cygwin  | 1 -
src/makefiles/Makefile.freebsd | 2 --
src/makefiles/Makefile.hpux    | 5 -----
src/makefiles/Makefile.linux   | 2 --
src/makefiles/Makefile.netbsd  | 2 --
src/makefiles/Makefile.openbsd | 2 --
src/makefiles/Makefile.solaris | 6 +-----
src/makefiles/Makefile.win32   | 1 -
src/template/aix               | 5 +++++
src/template/cygwin            | 3 +++
src/template/darwin            | 3 +++
src/template/freebsd           | 3 +++
src/template/hpux              | 7 +++++++
src/template/linux             | 3 +++
src/template/netbsd            | 3 +++
src/template/openbsd           | 4 ++++
src/template/solaris           | 9 +++++++++
src/template/win32             | 3 +++
21 files changed, 53 insertions(+), 22 deletions(-)