On linux we build with defined _GNU_SOURCE.

We must perform system tests with _GNU_SOURCE too!

cmake/os/Linux.cmake

@@ -22,6 +22,7 @@ INCLUDE(CheckSymbolExists)
 SET(TARGET_OS_LINUX 1)
 SET(HAVE_NPTL 1)
 SET(_GNU_SOURCE 1)
+SET(CMAKE_REQUIRED_DEFINITIONS ${CMAKE_REQUIRED_DEFINITIONS} -D_GNU_SOURCE=1)
 
 # Fix CMake (< 2.8) flags. -rdynamic exports too many symbols.
 FOREACH(LANG C CXX)