CMake: Don't compile with -D_GNU_SOURCE

We do not want to compile with GNU-specific extensions, as we want the
code to be as portable as possible.

Thanksfully we don't actually need _GNU_SOURCE to be defined, removing
it only triggers an error in network-unix.c about an undefined
structure, which we can fix by including the right header.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

CMakeLists.txt

@@ -120,9 +120,6 @@ endif()
 if(APPLE)
 	#full Single Unix Standard v3 (SUSv3) conformance (the Unix API)
 	add_definitions(-D_DARWIN_C_SOURCE)
-elseif (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
-	set(CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE=1")
-	add_definitions(-D_GNU_SOURCE=1)
 elseif (${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD|DragonFly|OpenBSD|NetBSD")
 	set(CMAKE_REQUIRED_DEFINITIONS "-D__BSD_VISIBLE")
 	add_definitions(-D__BSD_VISIBLE=1)

network-unix.c

@@ -17,6 +17,7 @@
 #include <poll.h>
 #include <string.h>
 #include <sys/socket.h>
+#include <sys/time.h>
 #include <unistd.h>
 
 int set_blocking_mode(int fd, bool blocking)