[Makefile] Concetenate and Minify CSS via sed and tr Command

SOURCE_CSS_FILES=$(filter-out %.min.css, $(wildcard *.css))
MINIFIED_CSS=app.min.css

minify: concat
      @# remove css comments
      sed -r ':a; s%(.*)/\*.*\*/%\1%; ta; /\/\*/ !b; N; ba' -i $(MINIFIED_CSS)
      @# remove leading spaces and tabs
      sed 's/^\s*//' -i $(MINIFIED_CSS)
      @# remove trailing spaces, tabs, and newline
      sed 's/\s*$$//' -i $(MINIFIED_CSS)
      @# remove newline
      tr --delete '\n' < $(MINIFIED_CSS) > tmp.css
      mv tmp.css $(MINIFIED_CSS)

concat:
      cat $(SOURCE_CSS_FILES) > $(MINIFIED_CSS)