make: use '"filename" line 123' for locations instead of 'filename:123'

The format 'filename:123' is commonly used for identifying a location in
a file. Text editors recognize it and allow quick navigation through it.
The previous format was specific to make and would have required custom
support in editors.

The new format was already used in stack traces, except for the first
line. Now all lines use the same format.

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: 1.3.2;
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.

make(1): fix off-by-one error in Var_Parse for dynamic variables

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