make: replace "Malformed conditional" with "Variable is undefined"

Evaluating a conditional such as ".if ${UNDEF}" produced a "Malformed
conditional" error, even though the form of the conditional was correct,
it was the content of the variable that was wrong.

Replace this message with the more accurate "Variable is undefined",
revealing which of the possibly many variables is actually undefined.

make: in error message about conditionals, use single quotes

Single quotes are used less often in the conditionals themselves, which
leads to fewer confusions.

branches: 1.8.2;
tests/make: replace 'variable expressions' with 'expressions'

tests/make: force line-based diagnostics to be listed in the tests

This way, contradictions between the intended output and the actual
output are closer together and have a better chance of being spotted.

tests/make: clean up comments, extend a few tests

make: update constant names in comments

No functional change.

make: rename context and ctxt to scope

This continues the previous commit, in which VAR_GLOBAL, VAR_INTERNAL
and VAR_CMDLINE were renamed.

Renaming the variable 'ctxt' was trivial since that word is used nowhere
else. In the comments though, each occurrence of the word 'context' had
to be checked individually since the word 'context' was not only used
for referring to a variable scope. It is also used to distinguish
different situations where characters are escaped in a certain way
('parsing context') and in a few other expressions.

make(1): add test for dynamic variable with modifiers in global context

make(1): add test for dynamic variable with modifiers in Var_Parse

make(1): add test for off-by-one error in Var_Parse