[Bug #19831] Remove duplicate library warning

When building on macOS, we get a warning about duplicate libraries

    ld: warning: ignoring duplicate libraries: '-ldl', '-lobjc', '-lpthread'

To fix it, we now append $(MAINLIBS) to LIBRUBYARG_SHARED (when shared
is enabled), matching what's already done for LIBRUBYARG_STATIC. And we
remove the now-redundant $(MAINLIBS) from the $(PROGRAM) link line,
since $(LIBRUBYARG) carries it in both cases.

Author: etiennebarrie

configure.ac

@@ -4342,6 +4342,9 @@ AS_IF([test -n "${LIBS}"], [
     MAINFLAGS=`echo " $MAINLIBS " | sed "s|$libspat"'||;s/^ *//;s/ *$//'`
 ])
 LIBRUBYARG_STATIC="${LIBRUBYARG_STATIC} \$(MAINLIBS)"
+AS_IF([test "$enable_shared" = yes], [
+    LIBRUBYARG_SHARED="${LIBRUBYARG_SHARED} \$(MAINLIBS)"
+])
 CPPFLAGS="$CPPFLAGS "'$(DEFS) ${cppflags}'
 AS_IF([test -n "${cflags+set}"], [
     cflagspat=`eval echo '"'"${cflags}"'"' | sed 's/[[][|.*]]/\\&/g;s/^  */ /;s/^  *$/ /'`

template/Makefile.in

@@ -314,7 +314,7 @@ miniruby$(EXEEXT):
 $(PROGRAM):
 		@$(RM) $@
 		$(ECHO) linking $@
-		$(Q) $(PURIFY) $(CC) $(EXE_LDFLAGS) $(XLDFLAGS) $(MAINOBJ) $(EXTOBJS) $(LIBRUBYARG) $(MAINLIBS) $(EXTLIBS) $(OUTFLAG)$@
+		$(Q) $(PURIFY) $(CC) $(EXE_LDFLAGS) $(XLDFLAGS) $(MAINOBJ) $(EXTOBJS) $(LIBRUBYARG) $(EXTLIBS) $(OUTFLAG)$@
 		$(Q) $(POSTLINK)
 
 $(PROGRAM): @XRUBY_LIBPATHENV_WRAPPER@