Explicitly disallow paths with whitespace

It didn't work anyway even before this commit.

Workarounds #490.

Makefile

@@ -176,6 +176,10 @@ DOWNLOAD_PKG_ARCHIVE = \
           echo; \
           rm -f '$(PKG_DIR)/$($(1)_FILE)'; )
 
+ifneq ($(words $(PWD)),1)
+    $(error GNU Make chokes on paths with spaces)
+endif
+
 ifeq ($(IGNORE_SETTINGS),yes)
     $(info [ignore settings.mk])
 else ifeq ($(wildcard $(PWD)/settings.mk),$(PWD)/settings.mk)

index.html

@@ -184,6 +184,11 @@
     the first packages are built.
     </p>
 
+    <p>
+    Due to limitations of GNU Make, the path of MXE is not allowed
+    to contain any whitespace characters.
+    </p>
+
     <h3 id="tutorial-2">Step 2: System-wide Installation (optional)</h3>
 
     <p>