Simplify Makefile.

How I dealt with .d files previously was inspired by GNU make manual
(http://www.gnu.org/software/make/manual/make.html#Automatic-Prerequisites),
but it can be easily simplified to a one line. So here it goes.

+ Note regarding my previous Makefile-related commit:
gperf invocation is purposely using redirection instead of --output-file
option to avoid useless html_unescape.h change when regenerated
(it would add --output-file=html_unescape.h in Command-line: comment).