Update documentation build to use saner version extraction

Patch by Igor Poretsky

Change-Id: I0c61d42dcac79b72c6b13d700732237a845f676e
diff --git a/manual/Makefile b/manual/Makefile
index 410d5a6..e1d3c1f 100644
--- a/manual/Makefile
+++ b/manual/Makefile
@@ -18,7 +18,7 @@
 .PHONY: all buildmanual clean objdir-check
 MANFILE := rockbox-build
 
-VERSION= `$(TOOLSDIR)/version.sh $(ROOTDIR)`
+VERSION ?= $(shell $(TOOLSDIR)/version.sh $(ROOTDIR))
 
 .PHONY: $(OBJDIR)/version.tex $(OBJDIR)/$(MANFILE).tex
 
diff --git a/manual/Makefile.pdflatex b/manual/Makefile.pdflatex
index c533e0f..ad58315 100644
--- a/manual/Makefile.pdflatex
+++ b/manual/Makefile.pdflatex
@@ -6,7 +6,7 @@
 HTOPTS1 := rockbox-html,charset=utf-8,early_
 HTOPTS2 := " -utf8 -cmozhtf"
 HTOPTS3 := "-dhtml/ -cvalidate"
-VERSION=`$(TOOLSDIR)/version.sh $(ROOTDIR)`
+VERSION ?= $(shell $(TOOLSDIR)/version.sh $(ROOTDIR))
 SPLITHTML ?= 2
 
 .PHONY: all cleaner buildmanual output