configure: Remove unneeded defines from checks

_GNU_SOURCE is already defined in QEMU_CFLAGS which
is passed to gcc in shell function compile_prog.

Removing the definition from several checks avoids compiler warnings
(which are now written to config.log).
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -1891,7 +1891,6 @@ fi
 utimens=no
 cat > $TMPC << EOF
 #define _ATFILE_SOURCE
-#define _GNU_SOURCE
 #include <stddef.h>
 #include <fcntl.h>
 
@@ -1909,7 +1908,6 @@ fi
 # check if pipe2 is there
 pipe2=no
 cat > $TMPC << EOF
-#define _GNU_SOURCE
 #include <unistd.h>
 #include <fcntl.h>
 
@@ -1927,7 +1925,6 @@ fi
 # check if accept4 is there
 accept4=no
 cat > $TMPC << EOF
-#define _GNU_SOURCE
 #include <sys/socket.h>
 #include <stddef.h>
 
@@ -1944,7 +1941,6 @@ fi
 # check if tee/splice is there. vmsplice was added same time.
 splice=no
 cat > $TMPC << EOF
-#define _GNU_SOURCE
 #include <unistd.h>
 #include <fcntl.h>
 #include <limits.h>