doc/Makefile: avoid spurious whitespace in environment variables

* doc/Makefile (%.pdf): Ensure that LANG and LC_ALL are set to "C",
  not "C   " (with trailing whitespace).

Make evaluation order produces spurious whitespace in variables when a
variable definition is followed by whitespace plus a comment.

doc/Makefile

@@ -54,8 +54,9 @@ clean-install:
 %:		%.texi org-version.inc
 	$(MAKEINFO) --no-split $< -o $@
 
-%.pdf:		LC_ALL=C	# work around a bug in texi2dvi
-%.pdf:		LANG=C		# work around a bug in texi2dvi
+# the following two lines work around a bug in some versions of texi2dvi
+%.pdf:		LC_ALL=C
+%.pdf:		LANG=C
 %.pdf:		%.texi org-version.inc
 	$(TEXI2PDF) $<
 %.pdf:		%.tex