Fix building with musl libc

POSIX says poll.h and signal.h should not be prefixed with sys/ (ie
poll.h instead of sys/poll.h)

limits.h also defines a TZNAME_MAX so we change name of the define for
spandsp.

libs/libscgi/src/include/scgi.h

@@ -116,7 +116,7 @@ typedef int scgi_filehandle_t;
 #include <sys/types.h>
 #include <sys/select.h>
 #include <netinet/tcp.h>
-#include <sys/signal.h>
+#include <signal.h>
 #include <unistd.h>
 #include <ctype.h>
 #endif