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_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/aa5bb828af5387e154a122b1b43ee873d92497a0

Modified Files
--------------
configure | 7 ++++---
configure.in | 9 ++++++---
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, 55 insertions(+), 26 deletions(-)