libevdev: disable runtime tests

Add new configure option '--disable-runtime-tests' and use it do
disable runtime tests which fail to compile on some uclibc
configurations.

Fixes [1], [2]:
  test-libevdev-init.c: In function 'test_set_clock_id':
  test-libevdev-init.c:493: error: 'CLOCK_MONOTONIC_RAW' undeclared (first use in this function)

and compile failure for !BR2_USE_MMU configuration:
  test-main.c:(.text+0x8): undefined reference to `_fork'

[1] http://autobuild.buildroot.net/results/0d0/0d0a987471ce8e9f76fc96e0ae8f0bfeadb45028/
[2] http://autobuild.buildroot.net/results/939/9392033f1b8d2c8e4e8791c90918dabbe28a7067/



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>