build: Disclaimer that regressions are unsupported for out-of-source builds.

configure

@@ -555,6 +555,8 @@ mkdir -p $P_PWD/doc
 mkdir -p $P_PWD/build
 mkdir -p $P_PWD/include
 mkdir -p $P_PWD/src
+mkdir -p $P_PWD/regressions
+cp $BUILD_DIR/regressions/Makefile.unsupported $P_PWD/regressions/Makefile &> /dev/null
 cp $BUILD_DIR/build/ck.build.$PROFILE $P_PWD/build/ck.build.$PROFILE &> /dev/null
 cp $BUILD_DIR/include/ck_md.h $P_PWD/include/ck_md.h &> /dev/null
 

regressions/Makefile.unsupported

@@ -0,0 +1,9 @@
+.PHONY: all clean check
+
+all:
+	@echo Regressions are currently unsupported for out-of-source builds
+
+clean: all
+
+check: all
+