pgsql: Build src/port files as a library with -fPIC, and use that in li

Build src/port files as a library with -fPIC, and use that in libpq.

libpq and ecpg need shared-library-friendly versions of assorted src/port/
and src/common/ modules. Up to now, they got those by symlinking the
individual source files and compiling them locally. That's baroque, and a
pain to maintain, and it results in some amount of duplicated compile work.
It might've made sense when only a couple of files were needed, but the
list has grown and grown and grown :-(

It makes more sense to have the originating directory build a third variant
of libpgport.a/libpgcommon.a containing modules built with $(CFLAGS_SL),
and just link that into the shared library. Unused files won't get linked,
so the end result should be the same.

This patch makes a down payment on that idea by having src/port/ build
such a library and making libpq use it. If the buildfarm doesn't expose
fatal problems with the approach, I'll extend it to the other cases.

Discussion: https://postgr.es/m/13022.1538003440@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ea53100d5671b5b243f77898b0b04d23c38b2820

Modified Files
--------------
src/interfaces/libpq/.gitignore | 21 ----------------
src/interfaces/libpq/Makefile | 41 +++++++++---------------------
src/port/.gitignore | 1 +
src/port/Makefile | 55 ++++++++++++++++++++++++++++++++---------
4 files changed, 55 insertions(+), 63 deletions(-)