CMake: Move include(CheckCSourceCompiles) before its macros are used

Move include(CheckCSourceCompiles) before any of the
check_c_source_compiles() macros is called.

This fixes a CMake error when compiling with WITH_NETWORK_GET_BUFFER
disabled.

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

CMakeLists.txt

@@ -296,9 +296,10 @@ if(WITH_NETWORK_BACKEND)
 	endif()
 
 	if(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+		include(CheckCSourceCompiles)
+
 		option(WITH_NETWORK_GET_BUFFER "Enable experimental zero-copy transfers" OFF)
 		if (WITH_NETWORK_GET_BUFFER)
-			include(CheckCSourceCompiles)
 			check_c_source_compiles("#define _GNU_SOURCE=1\n#include <fcntl.h>\nint main(void) { return O_TMPFILE; }"
 				HAS_O_TMPFILE)