doc build: use unique Makefile variable to control temp install

doc/src/sgml/Makefile

@@ -17,6 +17,8 @@
 # to want to use.
 html:
 
+NO_TEMP_INSTALL=yes
+
 subdir = doc/src/sgml
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global

src/Makefile.global.in

@@ -303,7 +303,7 @@ check: temp-install
 
 .PHONY: temp-install
 temp-install:
-ifdef GENERATED_SGML # not needed for SGML 'make check'
+ifndef NO_TEMP_INSTALL
 ifeq ($(MAKELEVEL),0)
 	rm -rf '$(abs_top_builddir)'/tmp_install
 	$(MAKE) -C '$(top_builddir)' DESTDIR='$(abs_top_builddir)'/tmp_install install