doc/Makefile.am: use ` instead of shell to execute commands to avoid early execution (i.e before file timestamp file exists)

doc/Makefile.am

@@ -51,11 +51,11 @@ chapters/version.texi:
 	@for f in $(starpu_TEXINFOS) ; do \
                 if test -f $(top_srcdir)/doc/$$f ; then stat --format=%Y $(top_srcdir)/doc/$$f ; fi \
         done | sort -r | head -1 > timestamp
-	@LC_ALL=C date --date=@$(shell cat timestamp) +"%d %B %Y" > timestamp_updated
-	@LC_ALL=C date --date=@$(shell cat timestamp) +"%B %Y" > timestamp_updated_month
-	@echo "@set UPDATED " $(shell cat timestamp_updated) > $(top_srcdir)/doc/chapters/version.texi
-	@echo "@set UPDATED-MONTH" $(shell cat timestamp_updated_month) >> $(top_srcdir)/doc/chapters/version.texi
-	@echo "@set EDITION $(VERSION)" >> $(top_srcdir)/doc/chapters/version.texi
+	@LC_ALL=C date --date=@`cat timestamp` +"%d %B %Y" > timestamp_updated
+	@LC_ALL=C date --date=@`cat timestamp` +"%B %Y" > timestamp_updated_month
+	@echo "@set UPDATED " `cat timestamp_updated` > $(top_srcdir)/doc/chapters/version.texi
+	@echo "@set UPDATED-MONTH" `cat timestamp_updated_month` >> $(top_srcdir)/doc/chapters/version.texi
+	@echo "@set EDITION $(VERSION)" >> $(top_srcdir)/doc/chapters/version.texi && \
 	@echo "@set VERSION $(VERSION)" >> $(top_srcdir)/doc/chapters/version.texi
 
 #$(top_srcdir)/doc/starpu.texi: vector_scal_c.texi vector_scal_cuda.texi vector_scal_opencl.texi vector_scal_opencl_codelet.texi