Define unshare() macro to avoid dependence on __GNU_SOURCE

unshare() is a linux syscall that we need. Unfortunately, access to it
depends on __GNU_SOURCE, which we would like to avoid. As such, we
define a macro to invoke it by its syscall code. This is necessary to
avoid the following warning from Clang:

implicit declaration of function 'unshare' is invalid in C99
[-Wimplicit-function-declaration]

Signed-off-by: Richard Yao <ryao@gentoo.org>

src/test/test-udev.c

@@ -34,6 +34,14 @@
 
 #include "udev.h"
 
+#ifndef SYS_unshare
+#error "libc fails to set SYS_unshare: please file a bug report with eudev"
+#endif
+
+#ifndef unshare
+#define unshare(__X) syscall(SYS_unshare, __X)
+#endif
+
 void udev_main_log(struct udev *udev, int priority,
                    const char *file, int line, const char *fn,
                    const char *format, va_list args) {}