share/mk/: Reduce rebuilds, and simplify makefile code

I've verified that the build system produces the same amount of files
with 'make -R -j24 -k lint build-all check'.  This was just to make sure
there are no silent regressions.

	$ diff -u \
		<(find ~/src/linux/man-pages/man-pages/contrib/.tmp/ -type f \
			| sed s/contrib/X/ \
			| sort) \
		<(find ~/src/linux/man-pages/man-pages/main/.tmp/ -type f \
			| sed s/main/X/ \
			| sort);
	--- /dev/fd/63	2025-09-11 10:44:37.386884907 +0200
	+++ /dev/fd/62	2025-09-11 10:44:37.386884907 +0200
	@@ -3,7 +3,7 @@
	 /home/alx/src/linux/man-pages/man-pages/X/.tmp/fonts/devpdf/TinosR
	 /home/alx/src/linux/man-pages/man-pages/X/.tmp/fonts/devpdf/TinosR.afm
	 /home/alx/src/linux/man-pages/man-pages/X/.tmp/fonts/devpdf/download
	-/home/alx/src/linux/man-pages/man-pages/X/.tmp/man-pages-6.15-85-gbd98df418.pdf
	+/home/alx/src/linux/man-pages/man-pages/X/.tmp/man-pages-6.15-77-g1d0de88eb.pdf
	 /home/alx/src/linux/man-pages/man-pages/X/.tmp/man/man1/diffman-git.1
	 /home/alx/src/linux/man-pages/man-pages/X/.tmp/man/man1/diffman-git.1.cat
	 /home/alx/src/linux/man-pages/man-pages/X/.tmp/man/man1/diffman-git.1.cat.grep

The file contents are slightly different, but that's because for some
reason groff(1) is not reproducible; I've checked some samples, and the
differences seem trivial, and unrelated to these build-system changes.
I've reported to the groff@ mailing list the lack of reproducibility of
groff(1).

Signed-off-by: Alejandro Colomar <alx@kernel.org>