gpio: add feature test macro for POLLRDNORM
gpio: add feature test macro for POLLRDNORM

The code needs _XOPEN_SOURCE for some poll() flags.

Fix build with uclibc:
src/gpio.c: In function gpio_poll_multiple»:
src/gpio.c:242:61: error: «POLLRDNORM» undeclared (first use in this function)
(POLLPRI | POLLERR) : (POLLIN | POLLRDNORM);
                                ^

closes #24.

Signed-off-by: Joris Offouga <91995a9af358bb810cb00242dd57bee1906852f2@gmail.com>
Signed-off-by: Vanya A. Sergeev <7a38d8cbd20d9932ba948efaa364bb62651d5ad4@sergeev.io>

vsergeev/c-periphery