Check that EWOULDBLOCK is the same as EAGAIN

It certainly is everywhere on Linux, but as a courtesy
to people doing some strange cross-compilation, check
that the assumption holds.

src/shared/util.c

@@ -94,6 +94,9 @@
 #include "def.h"
 #include "sparse-endian.h"
 
+/* Put this test here for a lack of better place */
+assert_cc(EAGAIN == EWOULDBLOCK);
+
 int saved_argc = 0;
 char **saved_argv = NULL;