In the same way the editor reports undefined types/variables/macros/etc. in a C or C++ source file, it should also do the same for makefile/Makefile files.

This is especially useful when modifying an old or crafting a new makefile, because make silently fails undefined macros and simply places an empty string there.  This can cause strange behavior when the makefile is run that can be hard to identify as lines go scrolling off the screen at 100 miles an hour.  Even spotting it in captured output can be difficult at times.

Just flagging it with a red underline, as is done in the C source code editor would greatly enhance it's usefulness in editing a makefile.

A red indicator for the line that says why it is flagged would also be expected.

I did notice that a few times when I used $< that it knew it was illegal, but the only indication is if you put the insertion point right after the '<', in which case it turns it red.  But this should also be underlined as illegal and have a red indicator explaining why on the left side.