package/exim: fix build with ccache

package/exim: fix build with ccache

When using ccache to build the exim package, the HOSTCC value contains
spaces, that are incorrectly interpreted by exim's Makefilei, which uses
the first word of ${CC} to test compiler options. This breaks the build
with "unrecognized option" ccache errors.

Fix that by wrapping the HOSTCC variable in double quotes, as it is done
for other variables that follow.

Signed-off-by: Alejandro González <alejandro.gonzalez.correo@gmail.com>
[yann.morin.1998@free.fr: slight rewording of commit log]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>