build: make the static-file build repeatable

Two things kept the same source from producing the same image.

fileadder walked the input directory with readdir(), which returns entries
in whatever order the filesystem hands them over. The same tree therefore
embedded the files in a different order on a different filesystem, and the
image differed byte for byte. scandir() with alphasort() fixes the order.

version.h was listed in .PHONY, which makes it permanently out of date and
propagates that to everything including it, so a plain build recompiled the
four largest translation units even when nothing had changed. Dropping it
from .PHONY alone would go too far the other way: with no prerequisites the
recipe would never run again and the version string would freeze at its
first value. A FORCE prerequisite keeps the recipe running every time, and
the generated header is only moved into place when its content actually
differs, so the timestamp survives an unchanged rebuild.

Measured on this tree: with FORCE the version follows GIT_VERSION and an
unchanged rebuild leaves version.h untouched; without it the header keeps
the value from the first build.

Makefile

@@ -112,11 +112,14 @@ html_min: $(HTML)
 html_data.c html_data.h &: $(HTML) | tools html_min
 	tools/output/fileadder -a $(HTML_LOCATION) -s $(IMAGESIZE) -b BANK1 -z -d $(HTML_MIN) -p html_data
 
-$(VERSION_HEADER):
+$(VERSION_HEADER): FORCE
 	@printf '%s\n' "#ifndef VERSION_H" "#define VERSION_H" \
 		"#define VERSION_SW \"$(VERSION_EXTENSION)\"" \
 		"#define BUILD_DATE \"$(BUILD_DATE)\"" \
-		"#endif" > $(VERSION_HEADER)
+		"#endif" > $(VERSION_HEADER).tmp
+	@cmp -s $(VERSION_HEADER).tmp $(VERSION_HEADER) \
+		&& rm -f $(VERSION_HEADER).tmp \
+		|| mv $(VERSION_HEADER).tmp $(VERSION_HEADER)
 
 httpd: html_data.h
 
@@ -166,7 +169,7 @@ $(BUILDDIR)/rtlplayground-$(FILENAME_EXTENSION).bin: $(BUILDDIR)/rtlplayground.i
 	tools/output/crc_calculator -u $@
 	ln -sf $(MACHINE)/rtlplayground-$(FILENAME_EXTENSION).bin output/rtlplayground.bin
 
-.PHONY: clean distclean all $(SUBDIRS) $(SUBDIRSCLEAN) $(VERSION_HEADER) create_build_dir
+.PHONY: clean distclean all $(SUBDIRS) $(SUBDIRSCLEAN) create_build_dir
 
 .PHONY:
 machine_check: