Fix shell error on Solaris

Apparently, the Bourne shell on Solaris doesn't like "for" loops with an
empty list, so have "make" skip the loop in that case.

src/Makefile.global.in

@@ -311,7 +311,7 @@ ifeq ($(MAKELEVEL),0)
 	$(MKDIR_P) '$(abs_top_builddir)'/tmp_install/log
 	$(MAKE) -C '$(top_builddir)' DESTDIR='$(abs_top_builddir)'/tmp_install install >'$(abs_top_builddir)'/tmp_install/log/install.log 2>&1
 endif
-	for extra in $(EXTRA_INSTALL); do $(MAKE) -C '$(top_builddir)'/$$extra DESTDIR='$(abs_top_builddir)'/tmp_install install >>'$(abs_top_builddir)'/tmp_install/log/install.log 2>&1 || exit; done
+	$(if $(EXTRA_INSTALL),for extra in $(EXTRA_INSTALL); do $(MAKE) -C '$(top_builddir)'/$$extra DESTDIR='$(abs_top_builddir)'/tmp_install install >>'$(abs_top_builddir)'/tmp_install/log/install.log 2>&1 || exit; done)
 
 PROVE = @PROVE@
 PG_PROVE_FLAGS = -I $(top_srcdir)/src/test/perl/