Guard individual compatibility header prototypes.

This is to avoid redefining prototypes from the libc headers. Also,
simplify the autoconf function checks and remove some copy/paste
errors checking for 'write'.

ok wouter@

include/unistd.h

@@ -3,7 +3,12 @@
 #ifndef LIBCRYPTOCOMPAT_UNISTD_H
 #define LIBCRYPTOCOMPAT_UNISTD_H
 
+#ifdef NO_GETENTROPY
 int getentropy(void *buf, size_t buflen);
+#endif
+
+#ifdef NO_ISSETUGID
 int issetugid(void);
+#endif
 
 #endif