Support multiple TARGETs

[kingjon3377]

In my current project, I'm building two PDFs from essentially the same source but using two incompatible document-types. After I reread Makefile.include, I got this to more-or-less work with the following:

TARGET=poems
PDFTARGETS=poems-alt
include ~/src/pdflatex-makefile/Makefile.include

But I would like to be able to simply say

TARGETS=poems poems-alt
include ~/src/pdflatex-makefile/Makefile.include

[ransford]

Good idea!