fixes in manual2pdf

- option -i without argument is a GNU extension
- exit script if fix_latxmath returns with an error

manual2pdf

@@ -20,9 +20,9 @@ rm -f *.aux *.bbl *.cb* *.lo* *.idx *.toc *.tex *.xml *.glo *.4tc *.dvi *.tmp *.
 # we have to set the revdate here. Otherwise asciidoctor uses
 # the timestamp of Manual.asciidoc.
 declare revdate=$(git log -1 --date=format:'%Y-%m-%d' --format=%cd)
-sed -i "s/@REVDATE@/${revdate}/" Manual.asciidoc
+sed -i.bak "s/@REVDATE@/${revdate}/" Manual.asciidoc
 
-"${mydir}/fix_latexmath"
+"${mydir}/fix_latexmath" || exit 3
 
 if [[ "${create_tex_output}" == "no" ]]; then
 	asciidoctor -b docbook5 -d book "${input_fname}" || exit 1