python, meson: Assert that deps are present for -Dpython=true

python, meson: Assert that deps are present for -Dpython=true

The -Dpython option takes 3 values: auto, true, or false. For
"auto", the Python bindings will be built if all the dependencies
are met and will be skipped in not. For "true", the Python bindings
MUST be built and therefore missing dependencies need to be treated
as an error. For "false", the Python bindings won't be built whether
the dependencies are met or not.

Currently, with -Dpython=true, if a dependency required to build the
Python bindings is missing, meson silently skips it (i.e. it behaves
like "-Dpython=auto").

Signed-off-by: Martin Belanger <martin.belanger@dell.com>