Otherwise, it gets executed with /bin/sh which on Solaris does not support the
$(...) construct. This causes 'gmake check' to fail. For example:

../src/check_headers: $(dirname ../src/check_headers): does not exist

Calling check_headers with $(SHELL) explicitly makes use of CONFIG_SHELL.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>