dhcpcd: fix build with ccache

After the bump to dhcpcd 6.4.0, building dhcpcd with ccache fails at the
configure step:

Using compiler .. <buildroot>/output/host/usr/bin/ccache <buildroot>/output/host/usr/bin/i686-pc-linux-gnu-gcc
<buildroot>/output/host/usr/bin/ccache <buildroot>/output/host/usr/bin/i686-pc-linux-gnu-gcc is not an executable
make: *** [<buildroot>/output/build/dhcpcd-6.4.0/.stamp_configured] Error 1

This patch backports an upstream patch to fix this issue.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>