diff --git a/project/config/cfgdefs.in b/project/config/cfgdefs.in
index e69de29..c63687f 100644
--- a/project/config/cfgdefs.in
+++ b/project/config/cfgdefs.in
@@ -0,0 +1,10 @@
+# cfgdefs.mk: build project definitions and variable assignments,
+#   generated by the project-specific cfgdefs.sh.
+
+# changes made to this file will be gone the next time
+#   you run ./configure.
+
+# for persistent post-configure, ad-hoc changes to the
+#   build project, use usrdefs.mk instead.
+
+CFLAGS_CONFIG           += -D_PATH_DEFSYSPATH=\"@cfgdefs_default_sys_path@\"
diff --git a/project/config/cfgdefs.sh b/project/config/cfgdefs.sh
index 2ff7631..a4e859f 100644
--- a/project/config/cfgdefs.sh
+++ b/project/config/cfgdefs.sh
@@ -17,10 +17,27 @@
 
 for arg ; do
 	case "$arg" in
+		--with-default-sys-path=*)
+			cfgdefs_default_sys_path=${arg##*=}
+			;;
 		*)
 			error_msg ${arg#}: "unsupported config argument."
 			exit 2
 	esac
+
+	case "${cfgdefs_default_sys_path:-empty}" in
+		/*)
+			;;
+
+		empty)
+			error_msg "$mb_script: --with-default-sys-path: empty paths are not allowed."
+			exit 2
+			;;
+
+		*)
+			error_msg "$mb_script: --with-default-sys-path: relative paths are not allowed."
+			exit 2
+	esac
 done
 
 
@@ -121,7 +138,12 @@ cfgdefs_bootstrap()
 
 cfgdefs_output_custom_defs()
 {
-	cat "$mb_project_dir/project/config/cfgdefs.in" > cfgdefs.mk
+	cfgdefs_default_sys_path="${cfgdefs_default_sys_path:-/usr/share/mk}"
+
+	sed                                                                       \
+		-e 's#@cfgdefs_default_sys_path@#'"$cfgdefs_default_sys_path"'#g' \
+			"$mb_project_dir/project/config/cfgdefs.in"               \
+		> cfgdefs.mk
 }
 
 
diff --git a/project/extras.mk b/project/extras.mk
index a5357a0..79b0dd2 100644
--- a/project/extras.mk
+++ b/project/extras.mk
@@ -1,5 +1,4 @@
-SYS_MK_DIR               = $(DATADIR)/mk
-CFLAGS_CONFIG           += -D_PATH_DEFSYSPATH=\"$(SYS_MK_DIR)\"
+SYS_MK_DIR = $(DATADIR)/mk
 
 MK_FILES = \
 	$(SOURCE_DIR)/mk/ChangeLog \