README: update the building section
README: update the building section

Commit 6876721f7407 ("build: use a more standard autogen.sh") made
the supplied autogen execute ./configure unless the NOCONFIGURE
environment variable is set. Reflect that in the README file.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod