frame FTBFS on non-Linux architecture

[Stephen M. Webb]

The source package fails to build from source on Debian hurd-* and kfreebsd-*
arches with the following error.

/bin/bash ../libtool --tag=CXX --mode=compile g++ -DHAVE_CONFIG_H -I. -I.. -I../include/ -I../src -Wall -Werror -DBUILDING_FRAME -D_FORTIFY_SOURCE=2 -std=c++0x -fvisibility=hidden -fvisibility=hidden -I/usr/include/pixman-1 -I/usr/include/xorg -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -c -o
handle.lo handle.cpp
libtool: compile: g++ -DHAVE_CONFIG_H -I. -I.. -I../include/ -I../src -Wall -Werror -DBUILDING_FRAME -D_FORTIFY_SOURCE=2 -std=c++0x -fvisibility=hidden -fvisibility=hidden -I/usr/include/pixman-1 -I/usr/include/xorg -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -c handle.cpp -fPIC -DPIC -o .libs/handle.o
handle.cpp:25:25: fatal error: sys/eventfd.h: No such file or directory
compilation terminated.
make[3]: *** [handle.lo] Error 1