SetDirectory["C:\\drorbn\\AcademicPensieve\\Classes\\1617-257-AnalysisII"]

Make::usage = "Make[targets, sources, action] makes a target, or a list of targets, given sources, or a list of sources, in the style of the unix 'make' command.";
Make[target_String, sources_, action_Hold] := Make[Evaluate@{target}, sources, action];
Make[targets_, source_String, action__Hold] := Make[targets, Evaluate@{source}, action];
Make[targets_List, sources_List, action_Hold] := Module[{},
If[
(And @@ ((FileType[#] =!= None)& /@ sources)) &&
Or[
Or @@ ((FileType[#]===None)& /@ targets),
Min[AbsoluteTime[FileDate[#]]& /@ targets] < Max[AbsoluteTime[FileDate[#]]& /@ sources]
],
Print["Making ",targets, " ..."];
ReleaseHold[action]
]
];

?Make

{target, source} = {"Hour44Handout.png", "Hour44Handout.pdf"};
Make[target, source, Hold[
Read["!convert -density 300 -background white -alpha remove "<>source<>" "<>target];
MakeThumb[target];
]]

{
"TitleNotes" -> "This is my personal notebook for my 2016/17 <a href=http://drorbn.net/?title=1617-257>MAT 257 Analysis II</a> class (2017 part). <span style=\"color: red;\">Use at your own risk.</span><p>",
"CleanTeX" -> True,
"ImageComments" -> {
"JellyCube.jpg" -> "A Jelly Cube from <a class=external href=\"http://goronic.deviantart.com/art/Jelly-Cube-559481292\">http://goronic.deviantart.com/art/Jelly-Cube-559481292</a>."
}
}