build: check for realpath() in configure.ac
build: check for realpath() in configure.ac

Core library is now using realpath(). Check its availability in
configure.ac.

Signed-off-by: Bartosz Golaszewski <e5d45fddd9040473413b909b802f629a971ad04c@baylibre.com>

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