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: in error message about conditionals, use single quotes

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

branches: 1.7.2;
tests/make: test very small and very large numbers in conditions

make: add more details to error message for numeric comparison

Before:
String comparison operator must be either == or !=

After:
Comparison with '>=' requires both operands 'no' and '10' to be numeric

Noticed by martin@ in pkgsrc/textproc/py-pygments.

tests/make: test spaces around numbers in comparisons

Just to see whether it is possible to write a conditional in the form
${ ${A} < ${B} :? ${A} : ${B} }, that is, with leading and trailing
whitespace, to make it easier for humans to read the code.

It's not possible, the result of this computation cannot be used in
further numeric comparisons, at least not in .if directives. Leading
space would work, but trailing space wouldn't.

On the other hand, they would work in expressions of the form
${ ${A} < ${B} :? ... : ... } since in these, the condition is first
expanded and then parsed. But that is an implementation detail that is
not documented and it is also difficult to understand.

make(1): add test for wrong comparison operator in conditional

make(1): add test for Infinity and NaN

Neither is recognized by the make parser, which is good since the main
task of make is dependency resolution and text processing, not
calculating.

make(1): describe the purpose of each newly added unit test dummy

make(1): add dummies for fine-grained tests, one per single feature

The test names have been derived from the current manual page.

All these tests are dummies right now, and the code from the existing
tests will be moved into the new tests step by step.

This is done to prevent modmisc, escape, varmod-edge and varmisc from
growing without any bounds, and to reduce the side-effects of one test
to the others.