AIX: Fix missing libpq symbols by respecting SHLIB_EXPORTS.

We make each AIX shared library export all globals found in .o files
that originate in the library.  That doesn't include symbols acquired by
-lpgcommon_shlib.  That is good on average, but it became a problem for
libpq when commit e6afa891 moved five
official libpq API symbols into src/common.  Fix this by implementing
the SHLIB_EXPORTS mechanism for AIX, so affected libraries export the
same symbols that they export on Linux.  This reintroduces symbols
pg_encoding_to_char, pg_utf_mblen, pg_char_to_encoding,
pg_valid_server_encoding, and pg_valid_server_encoding_id.  Back-patch
to v13, where the aforementioned commit first appeared.  While a minor
release is usually the wrong time to add or remove symbol exports in
libpq or libecpg, we should expect users to want each documented symbol.

Tony Reix

Discussion: https://postgr.es/m/PR3PR02MB6396742E2FC3E77D37A920BC86C79@PR3PR02MB6396.eurprd02.prod.outlook.com

src/Makefile.shlib

@@ -329,7 +329,11 @@ $(shlib): $(OBJS) | $(SHLIB_PREREQS)
 	rm -f $(stlib)
 	$(LINK.static) $(stlib) $^
 	$(RANLIB) $(stlib)
+ifeq (,$(SHLIB_EXPORTS))
 	$(MKLDEXPORT) $(stlib) $(shlib) >$(exports_file)
+else
+	( echo '#! $(shlib)'; $(AWK) '/^[^#]/ {printf "%s\n",$$1}' $(SHLIB_EXPORTS) ) >$(exports_file)
+endif
 	$(COMPILER) -o $(shlib) $(stlib) -Wl,-bE:$(exports_file) $(LDFLAGS) $(LDFLAGS_SL) $(SHLIB_LINK)
 	rm -f $(stlib)
 	$(AR) $(AROPT) $(stlib) $(shlib)

src/port/README

@@ -28,5 +28,5 @@ applications.
 from libpgport are linked first.  This avoids having applications
 dependent on symbols that are _used_ by libpq, but not intended to be
 exported by libpq.  libpq's libpgport usage changes over time, so such a
-dependency is a problem.  Windows, Linux, and macOS use an export list to
-control the symbols exported by libpq.
+dependency is a problem.  Windows, Linux, AIX, and macOS use an export
+list to control the symbols exported by libpq.