configure.ac: fix `--with-linux-input` handling with upcoming autoconf-2.70

Upcoming autoconf-2.70 exposes deficiency in configure.ac:

$ autoconf-2.70_beta2 && ./configure --host=x86_64-pc-linux-gnu
./configure: line 1430: 5: Bad file descriptor
checking whether  is declared... ./configure: line 1432: ${+y}: bad

It happens because macros are called with parameters using insufficient quoting.

More details at https://lists.gnu.org/archive/html/bug-autoconf/2020-10/msg00027.html

The fix only amends --with-linux-input. Other cases of underquoting will need to be handled separately.

Fix-by: Zack Weinberg Signed-off-by: Sergei Trofimovich slyfox@gentoo.org