Stopgap help for the CPU-bound long hang in bug fdo#31716

As the hang seems to be causes by the code messing around in extension
help file directories, of which there are lots in a multi-lingual
build, just build en-US help for the Presenter Console
(a.k.a. presenter-screen) extension for now.

Why such a change is needed both here and in
solenv/inc/extension_helplink.mk (see commit
4e40f4ea586f240f38aa6856eda64619f505294a in the "bootstrap" repo) I
don't know, but that's how it seemed in my testing.

sdext/source/presenter/makefile.mk

@@ -252,7 +252,7 @@ PLATFORMID:=$(RTL_OS:l)_$(RTL_ARCH:l)
 
 COMPONENT_HELP= 								\
     $(ZIP1DIR)$/help/component.txt				\
-    $(foreach,l,$(alllangiso) $(ZIP1DIR)$/help$/$l$/com.sun.PresenterScreen-$(PLATFORMID)$/presenter.xhp)
+    $(ZIP1DIR)$/help$/en-US$/com.sun.PresenterScreen-$(PLATFORMID)$/presenter.xhp
 
 ZIP1DEPS=					\
     $(DESCRIPTION)			\