How to set up a conditional loop that chooses which compiler to use within the same document?

Question

I have a TeXlive 2019 installation with an unsolved problem that so far prevents me from compiling on this computer with LuaLaTeX, so I thought I could add to the preamble of the document that I'm working some code that allows me to choose to compile with LuaLaTeX and otherwise do it with pdfLaTeX.

I'm pretty sure I saw something like that years ago but I've been unable to find the solution I'm looking for.

Do any of you remember the solution I'm looking for? At the moment my MWE would be as follows:

\documentclass[fontsize=10pt,headings=big,headsepline=on,paper=7in:8.5in,DIV=calc,BCOR=5mm]{scrbook}
\usepackage[english]{babel}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX, Scale=MatchLowercase}
\usepackage{xunicode}
\usepackage{luatextra}
\setmainfont[
ItalicFont = SourceSerifPro,
ItalicFeatures={FakeSlant=0.2}
]{SourceSerifPro}
\setsansfont{SourceSansPro} 
\setmonofont{SourceCodePro}
\usepackage{blindtext}

\begin{document}
\Blindtext[3][2]
\end{document}

Answer 1

This version of the document works in lualatex, xelatex and pdflatex (and latex and with some warnings platex as well)

\documentclass[fontsize=10pt,headings=big,headsepline=on,paper=7in:8.5in,DIV=calc,BCOR=5mm]{scrbook}
\usepackage[english]{babel}
\usepackage{iftex}

\iftutex
% luatex or xetex
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX, Scale=MatchLowercase}
%\usepackage{xunicode}
%\usepackage{luatextra}
\setmainfont[
ItalicFont = SourceSerifPro,
ItalicFeatures={FakeSlant=0.2}
]{SourceSerifPro}
\setsansfont{SourceSansPro} 
\setmonofont{SourceCodePro}
\else
% pdftex
\fi
\usepackage{blindtext}

\begin{document}
\Blindtext[3][2]
\end{document}