Use (again) Perl concatenation for latexmk variables (refs sphinx-doc…

…#3719)

Indeed, with this latexmk will display something like

Running 'pdflatex --halt-on-error --interaction=batchmode  -recorder
"sphinx.tex"'

if `make latexpdf` was invoked with `LATEXOPTS="--halt... "`, which
is more useful console output than:

Running 'pdflatex $LATEXOPTS -recorder "sphinx.tex"'

as is the case without this commit.

sphinx/texinputs/latexmkjarc

@@ -1,4 +1,4 @@
-$latex = 'platex $LATEXOPTS -kanji=utf8 %O %S';
+$latex = 'platex ' . $ENV{'LATEXOPTS'} . ' -kanji=utf8 %O %S';
 $dvipdf = 'dvipdfmx %O -o %D %S';
 $makeindex = 'rm -f %D; mendex -U -f -d %B.dic -s python.ist %S || echo "mendex exited with error code $? (ignoring)" && : >> %D';
 add_cus_dep( "glo", "gls", 0, "makeglo" );

sphinx/texinputs/latexmkrc

@@ -1,7 +1,7 @@
-$latex = 'latex $LATEXOPTS %O %S';
-$pdflatex = 'pdflatex $LATEXOPTS %O %S';
-$lualatex = 'lualatex $LATEXOPTS %O %S';
-$xelatex = 'xelatex --no-pdf $LATEXOPTS %O %S';
+$latex = 'latex ' . $ENV{'LATEXOPTS'} . ' %O %S';
+$pdflatex = 'pdflatex ' . $ENV{'LATEXOPTS'} . ' %O %S';
+$lualatex = 'lualatex ' . $ENV{'LATEXOPTS'} . ' %O %S';
+$xelatex = 'xelatex --no-pdf ' . $ENV{'LATEXOPTS'} . ' %O %S';
 $makeindex = 'makeindex -s python.ist %O -o %D %S';
 add_cus_dep( "glo", "gls", 0, "makeglo" );
 sub makeglo {