Check for _YOSYS_ in yosys.h

kernel/yosys.h

@@ -58,6 +58,12 @@
 #include <string.h>
 #include <stdio.h>
 
+#ifndef _YOSYS_
+#  error It looks like you are trying to build Yosys with the config defines set. \
+         When building Yosys with a custom make system, make sure you set all the \
+	 defines the Yosys Makefile would set for your build configuration.
+#endif
+
 #ifdef YOSYS_ENABLE_TCL
 #  include <tcl.h>
 #endif