build: add a mechanism to tell whether the tree is configured yet

This means that bits of makefiles that depend on configure symbols and
might emit noisy errors otherwise can be surrounded with ifdef
CONFIGURED to suppress their execution when the makefile is initially
run (when all it does is configure itself and restart parsing, resetting
all its variables to the configured values).

We fix another noisy error while we're here: the initial build routinely
displays

GNUmakefile:111: /home/oranix/oracle/private/dtrace/build/config.mk: No such file or directory

because the tree is not yet configured and config.mk hasn't been
created.

This buys us nothing and is just noisy: suppress it.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

GNUmakefile

@@ -108,7 +108,7 @@ $(shell mkdir -p $(objdir))
 include Makeoptions
 include Makefunctions
 include Makeconfig
-include $(objdir)/config.mk
+-include $(objdir)/config.mk
 include Build $(sort $(wildcard */Build))
 -include $(objdir)/*.deps
 include Makerules

Makeconfig

@@ -89,6 +89,7 @@ $(CONFIG_H):
 
 $(CONFIG_MK):
 	echo '# This file is automatically generated.' > $(objdir)/config.mk
+	echo 'CONFIGURED := yes' >> $(objdir)/config.mk
 	cat $(objdir)/.config/*.mk >> $(objdir)/config.mk 2>/dev/null || true
 
 $(eval $(call check-symbol-rule,ELF_GETSHDRSTRNDX,elf_getshdrstrndx,elf))