meson.build: Add devenv for running from the build directory

This merge request adds support for running weston from the build directory using meson devenv.

Beyond the default for devenv environment variables for WESTON_MODULE_MAP and WESTON_DATA_DIR are also set.

To enter the environment:

meson devenv -C ./build

Check the environment

echo $WESTON_MODULE_MAP
echo $WESTON_DATA_DIR
which weston weston-smoke
ldd $(which weston)

Run wayland in the environment:

weston -S"wayland-5" &
sleep 1
WAYLAND_DISPLAY="wayland-5" weston-clickdot &