build: Use libroot_build on Haiku.

Previously we just used the system libroot, which of course meant
that when libroot's ABI changed, the build broke. Now we use the full
libroot_build that we do on non-Haiku platforms. The logic for "BeOS-compatible
but not Haiku" does not really apply anymore, so it has been gutted where
appropriate (and libhaikucompat has been decoupled from the build.)

The only caveat here is the change to Errors.h -- we really should be using
the system's one where I included the one from the tree, but for whatever
reason, GCC2 refused to handle the #include_next properly.

Fixes the build breakage of Haiku-on-Haiku by my prior commits (sorry).

headers/build/os/support/Errors.h

@@ -2,8 +2,8 @@
  * Copyright 2007-2014 Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  */
-#ifndef _ERRORS_H
-#define _ERRORS_H
+#ifndef _BUILD_ERRORS_H
+#define _BUILD_ERRORS_H
 
 
 #include <errno.h> /* build-specific overrides errno */
@@ -493,7 +493,8 @@
 
 	#undef errno
 	#define errno (*_haiku_build_errno())
-
+#elif defined(HAIKU_HOST_PLATFORM_HAIKU)
+#	include <../os/support/Errors.h>
 #endif	// ! BUILDING_HAIKU_ERROR_MAPPER
 
 #ifdef __cplusplus
@@ -508,4 +509,4 @@ extern int _haiku_to_host_error(int error);
 #endif
 
 
-#endif	/* _ERRORS_H */
+#endif	/* _BUILD_ERRORS_H */