Ensure dir is created before image BW conversion

Now that parallel build is enabled

book/makefile

@@ -95,15 +95,18 @@ epub/%.epub: %.tex $(src_all) cover/cover-page.xbb
 bw-book-epub:
 	mkdir -p bw-book-epub
 
+bw-book-epub/OEBPS: bw-book-epub
+	mkdir -p bw-book-epub/OEBPS
+
 copy_ebook_files: build_ebook | bw-book-epub
 	$(RSYNC) book-epub/ bw-book-epub/
 
 # We not convert SVG to B&W or lower res for now as they are super small
 # anyway
-bw-book-epub/OEBPS/%.jpg: %.jpg
+bw-book-epub/OEBPS/%.jpg: %.jpg | bw-book-epub/OEBPS
 	$(CONVERT_PIC) $< $(REDUCE_PIC) $@
 
-bw-book-epub/OEBPS/%.png: %.png
+bw-book-epub/OEBPS/%.png: %.png | bw-book-epub/OEBPS
 	$(CONVERT_PIC) $< $(REDUCE_PIC) $@
 
 epub/bw_book.epub: copy_ebook_files $(bw_images)