autoconf: Don't AC_SUBST() LD in configure

The only use of $(LD) in Makefiles is for AIX, to generate the export file for
the backend. We only support the system linker on AIX and we already hardcode
the path to a number of other binaries. Removing LD substitution will simplify
the upcoming meson PGXS compatibility.

While at it, add a comment why -r is used.

A subsequent commit will remove the determination of LD from configure as
well.

Discussion: https://postgr.es/m/20221005200710.luvw5evhwf6clig6@awork3.anarazel.de

src/Makefile.global.in

@@ -289,7 +289,6 @@ LDAP_LIBS_FE = @LDAP_LIBS_FE@
 LDAP_LIBS_BE = @LDAP_LIBS_BE@
 UUID_LIBS = @UUID_LIBS@
 LLVM_LIBS=@LLVM_LIBS@
-LD = @LD@
 with_gnu_ld = @with_gnu_ld@
 
 # It's critical that within LDFLAGS, all -L switches pointing to build-tree
@@ -316,8 +315,6 @@ LDFLAGS = $(LDFLAGS_INTERNAL) @LDFLAGS@
 LDFLAGS_EX = @LDFLAGS_EX@
 # LDFLAGS_SL might have already been assigned by calling makefile
 LDFLAGS_SL += @LDFLAGS_SL@
-LDREL = -r
-LDOUT = -o
 WINDRES = @WINDRES@
 X = @EXEEXT@
 

src/backend/Makefile

@@ -100,8 +100,14 @@ ifeq ($(PORTNAME), aix)
 postgres: $(POSTGRES_IMP)
 	$(CC) $(CFLAGS) $(call expand_subsys,$(OBJS)) $(LDFLAGS) $(LDFLAGS_EX) -Wl,-bE:$(top_builddir)/src/backend/$(POSTGRES_IMP) $(LIBS) -Wl,-brtllib -o $@
 
+# Linking to a single .o with -r is a lot faster than building a .a or passing
+# all objects to MKLDEXPORT.
+#
+# It looks alluring to use $(CC) -r instead of ld -r, but that doesn't
+# trivially work with gcc, due to gcc specific static libraries linked in with
+# -r.
 $(POSTGRES_IMP): $(OBJS)
-	$(LD) $(LDREL) $(LDOUT) SUBSYS.o $(call expand_subsys,$^)
+	ld -r -o SUBSYS.o $(call expand_subsys,$^)
 	$(MKLDEXPORT) SUBSYS.o . > $@
 	@rm -f SUBSYS.o