Not quite compiler independent, but we've never tried to be gcc independent

anyway.  This is at least less ugly than what was there before, and fixes
building all sources at once.

libbb/xfuncs.c

@@ -14,15 +14,7 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <fcntl.h>
-
-/* Since gcc always inlines strlen(), this saves a byte or two, but we need
- * the #undef here to avoid endless loop from #define strlen bb_strlen */
-#ifdef L_strlen
-#define BB_STRLEN_IMPLEMENTATION
-#endif
-
-#include "libbb.h"
-
+#include "busybox.h"
 
 #ifndef DMALLOC
 #ifdef L_xmalloc
@@ -182,10 +174,12 @@ void bb_xfflush_stdout(void)
 }
 #endif
 
+// GCC forces inlining of strlen everywhere, which is generally a byte
+// larger than calling a function, and it's called a lot so it adds up.
 #ifdef L_strlen
 size_t bb_strlen(const char *string)
 {
-	    return(strlen(string));
+	    return(__builtin_strlen(string));
 }
 #endif