Support of 'USE_PDFLATEX' variable and use 'basename' in "clean" commands

USE_PDFLATEX now determines whether to use 'latex' or 'pdflatex'
Also, "clean" commands use 'basename' now instead of 'sed'
Added 'help' and '.DEFAULT' targets

Added 'help' and '.DEFAULT' targets
Also now do a sanity check for the value of USE_PDFLATEX