share/mk/: Escape '#' in regexes

share/mk/: Escape '#' in regexes

GNU make(1) 4.2 seems to be interpreting those characters as the start
of a comment, so we need to escape them.  That seems to calm those old
versions of make(1), and doesn't affect negatively the newer ones, and
doesn't affect negatively grep(1) either.

Fixes: 35a780a99bd8 (2024-07-20; "share/mk/: CPPFLAGS: Only define _FORTIFY_SOURCE if it's not already defined")
Fixes: 2130162900ab (2024-11-03; "share/mk/, etc/shellcheck/: lint-sh: Add target to lint shell scripts")
Reported-by: Boris Pigin <boris.pigin@gmail.com>
Cc: Sam James <sam@gentoo.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>