conky: Fix building with CMake 3.4.0.

Regression introduced by 050bebb8

.

It's essentially an upgrade to CMake 3.4.0, which breaks the build
because it seems that in CMake 3.3.x, the check_include_files() command
was implicitly included (haven't found out about why exactly).

So we're now just adding an import for CheckIncludeFiles in addition to
CheckIncludeFile, so that we have both commands (the plural and the
singular variant) available.

My original goal was to use brndnmtthws/conky@3a574ba, but this breaks
the build as well, because check_include_files doesn't accept additional
compile flags.

However, this is needed if building with wireless support, because
including iwlib.h needs -D_GNU_SOURCE set and check_include_files
doesn't do that.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>