Incorrect plurals in string "%d source files given on command line"

[rffontenelle]

Describe the bug

sphinx/sphinx/builders/__init__.py

Lines 298 to 299 in 76110c3

	self.build(docnames, method='specific',
	summary=__('%d source files given on command line') % len(docnames))

The above code assumes the placeholder %d is plural, but it can be singular. This causes a weird rendered sentence of "1 source files...".

Besides, different languages have different plural rules, so the best would be to make this a ngettext / plural-form string so that the translator can translate correctly for number of plurals their language has.

How to Reproduce

1. Run: sphinx-build -M html . _build any_source_file
2. See an output like:

...
building [html]: 1 source files given on command line
...

Environment Information

Platform:              linux; (Linux-6.10.10-arch1-1-x86_64-with-glibc2.40)
Python version:        3.12.6 (main, Sep  8 2024, 13:18:56) [GCC 14.2.1 20240805])
Python implementation: CPython
Sphinx version:        8.0.2
Docutils version:      0.21.2
Jinja2 version:        3.1.4
Pygments version:      2.18.0

Sphinx extensions

No response

Additional context

No response