make: consistently use double quotes in error messages

Previously, some error messages used single quotes, very few used
backticks or parentheses, most already used double quotes.

While here, reword a few error messages to be clearer or more consistent
with related error messages.

make: use a common style for unexpected error messages

In enomem, report the actual error instead of a fixed ENOMEM.

make: add ":" to error message about unknown modifier

In the manual page, the modifiers are listed with a preceding ":", so
use the same pattern in the error message. This removes an inconsistency
between the error messages "Unknown modifier" and "Bad modifier".

make: omit redundant follow-up message "Malformed conditional"

If there is a parse error or an evaluation error during the conditional,
the conditional is already mentioned in the "while evaluating condition"
line, so there's no reason to repeat it once again.

Same for the "Bad condition" message from the '?:' modifier.

make: reduce line length in error messages

The error messages for deeply nested parse errors were hard to decipher,
due to the large amount of text. Split these messages into individual
lines, just as in the backtrace for .include files and .for loops. This
unified backtrace makes the output more uniform.

make: in error message about conditionals, use single quotes

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

make: add more context information to error messages

In case of a parse error or evaluation error, print the variable value
in addition to the variable name, to see the effects of previous
expression modifiers.

In nested make calls, print the current directory at the bottom of a
stack trace, as that information is otherwise hard to get in a parallel
build spanning multiple directories.

branches: 1.10.2;
make: provide more context information for parse/evaluate errors

tests/make: extend tests for include guards and empty expressions

tests/make: cover all code paths for the ':mtime' modifier

make: produce more accurate error message for invalid ':mtime' argument

tests/make: test non-integer fallback in ':mtime' modifier

tests/make: document bug in ':gmtime' with '%s'

varmod-mtime.mk: use :localtime for %s

Add a comment to explain why ${%s:L:localtime} should be used to get
a value that matches time(3).

make: be strict when parsing the argument of the ':mtime' modifier

tests/make: extend tests for the ':mtime' modifier

make: add :mtime to provide mtime of file

The value of the variable is passed to stat(2)
and st_mtime is new value.
An optional arg can be used if stat(2) fails, otherwise
the current time is used.

See varmod-mtime.mk for usage examples.