buildsys: Specify language to avoid searching for a C++ compiler

If no language (C, C++, fortran, etc) is specified, CMake will look for
both C and C++ compilers. It will fail if a C++ compiler is not found. Tell
CMake that we only need a C compiler.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
buildsys: Allow selecting drivers during cmake configuration

Although at the moment there is only the USB host driver, it still makes
sense to have an infrastructure for configuring which drivers to compile
in.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
buildsys: Only link to libusb when USB host driver is enabled

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>