Use exported symbols list on macOS for loadable modules as well

On macOS, when building with the make system, the exported symbols
list $(SHLIB_EXPORTS) was ignored.  This was probably not intentional,
it was probably just forgotten, since that combination has never
actually been used until now (for libpq-oauth).

The meson build system handles this correctly.  Also, other platforms
have been doing this correctly.

This fixes it.  It also does a bit of refactoring to make the code
match the layout for other platforms.

Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/c70ca32e-b109-460d-9810-6e23ebb4473f%40eisentraut.org

src/Makefile.shlib

@@ -112,7 +112,7 @@ ifeq ($(PORTNAME), darwin)
     ifneq ($(SO_MAJOR_VERSION), 0)
       version_link	= -compatibility_version $(SO_MAJOR_VERSION) -current_version $(SO_MAJOR_VERSION).$(SO_MINOR_VERSION)
     endif
-    LINK.shared		= $(COMPILER) -dynamiclib -install_name '$(libdir)/lib$(NAME).$(SO_MAJOR_VERSION)$(DLSUFFIX)' $(version_link) $(exported_symbols_list)
+    LINK.shared		= $(COMPILER) -dynamiclib -install_name '$(libdir)/lib$(NAME).$(SO_MAJOR_VERSION)$(DLSUFFIX)' $(version_link)
     shlib		= lib$(NAME).$(SO_MAJOR_VERSION)$(DLSUFFIX)
     shlib_major		= lib$(NAME).$(SO_MAJOR_VERSION)$(DLSUFFIX)
   else
@@ -122,7 +122,7 @@ ifeq ($(PORTNAME), darwin)
   BUILD.exports		= $(AWK) '/^[^\#]/ {printf "_%s\n",$$1}' $< >$@
   exports_file		= $(SHLIB_EXPORTS:%.txt=%.list)
   ifneq (,$(exports_file))
-    exported_symbols_list = -exported_symbols_list $(exports_file)
+    LINK.shared		+= -exported_symbols_list $(exports_file)
   endif
 endif