Fix for make unportability

88dad06b47 contains a make $(shell)
construct that apparently confuses older GNU make versions (possibly
because of the # inside the shell command?).  This construct, which
would allow # comments inside LINGUAS files, was adapted from gettext
recommendations, but we don't actually need that functionality, so
sidestep this whole issue by just using plain "cat".

In passing, make this code work with vpath.

src/nls-global.mk

@@ -29,7 +29,7 @@
 # existence checked by Makefile.global; otherwise we won't get here
 include $(srcdir)/nls.mk
 
-AVAIL_LANGUAGES := $(shell sed -e "/^#/d" -e "s/#.*//" po/LINGUAS)
+AVAIL_LANGUAGES := $(shell cat $(srcdir)/po/LINGUAS)
 
 # If user specified the languages he wants in --enable-nls=LANGUAGES,
 # filter out the rest.  Else use all available ones.