Remove logging.c from the shared library of src/common/

As fe0a1dc has proved, it is not a good concept to add to libpq
dependencies that would enforce the error output to a central logging
facility because it breaks the promise of reporting the error back to
an application in a consistent way, with the application to potentially
exit() suddenly if using pieces from for example jsonapi.c.  prairiedog
has allowed to report an actual design problem with fe0a1dc, but it will
not be around forever, so removing logging.c from libpgcommon_shlib is a
simple and much better long-term way to prevent any attempt to load the
central logging in libraries with general purposes.

Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20200928073330.GC2316@paquier.xyz

src/common/Makefile

@@ -88,16 +88,21 @@ OBJS_COMMON += sha2.o
 endif
 
 # A few files are currently only built for frontend, not server
-# (Mkvcbuild.pm has a copy of this list, too)
+# (Mkvcbuild.pm has a copy of this list, too).  logging.c is excluded
-OBJS_FRONTEND = \
+# from OBJS_FRONTEND_SHLIB (shared library) as a matter of policy,
+# because it is not appropriate for general purpose libraries such
+# as libpq to report errors directly.
+OBJS_FRONTEND_SHLIB = \
 	$(OBJS_COMMON) \
 	fe_memutils.o \
-	logging.o \
 	restricted_token.o \
 	sprompt.o
+OBJS_FRONTEND = \
+	$(OBJS_FRONTEND_SHLIB) \
+	logging.o
 
 # foo.o, foo_shlib.o, and foo_srv.o are all built from foo.c
-OBJS_SHLIB = $(OBJS_FRONTEND:%.o=%_shlib.o)
+OBJS_SHLIB = $(OBJS_FRONTEND_SHLIB:%.o=%_shlib.o)
 OBJS_SRV = $(OBJS_COMMON:%.o=%_srv.o)
 
 # where to find gen_keywordlist.pl and subsidiary files