Prevent against spaces in $MKIT_DIR

Found by running ShellCheck on .mk file ... by accident!

src/mkit.mk

 all: build
 
 build:
-	@$(MKIT_DIR)/make build
+	@"$(MKIT_DIR)"/make build
 
 manpages: build
-	@$(MKIT_DIR)/make build_manpages
+	@"$(MKIT_DIR)"/make build_manpages
 
 clean:
-	@$(MKIT_DIR)/make clean
+	@"$(MKIT_DIR)"/make clean
 
 debstuff: dist
-	@$(MKIT_DIR)/make debstuff
+	@"$(MKIT_DIR)"/make debstuff
 
 dist: clean
-	@$(MKIT_DIR)/make dist
+	@"$(MKIT_DIR)"/make dist
 
 rpmstuff: dist
-	@$(MKIT_DIR)/make rpmstuff
+	@"$(MKIT_DIR)"/make rpmstuff
 
 install: all
-	@$(MKIT_DIR)/make install
+	@"$(MKIT_DIR)"/make install
 
 release:
-	@$(MKIT_DIR)/make release
+	@"$(MKIT_DIR)"/make release
 
 release_x:
-	@$(MKIT_DIR)/make release_x
+	@"$(MKIT_DIR)"/make release_x
 
 release_y:
-	@$(MKIT_DIR)/make release_y
+	@"$(MKIT_DIR)"/make release_y
 
 release_z:
-	@$(MKIT_DIR)/make release_z
+	@"$(MKIT_DIR)"/make release_z
 
 uninstall:
-	@$(MKIT_DIR)/make uninstall
+	@"$(MKIT_DIR)"/make uninstall
 
 vbump:
-	@$(MKIT_DIR)/make vbump
+	@"$(MKIT_DIR)"/make vbump
 
 vbump_x:
-	@$(MKIT_DIR)/make vbump_x
+	@"$(MKIT_DIR)"/make vbump_x
 
 vbump_y:
-	@$(MKIT_DIR)/make vbump_y
+	@"$(MKIT_DIR)"/make vbump_y
 
 vbump_z:
-	@$(MKIT_DIR)/make vbump_z
+	@"$(MKIT_DIR)"/make vbump_z
 
 .PHONY: all clean dist rpmstuff install uninstall release release_x release_y release_z vbump vbump_x vbump_y vbump_z