gnu: pretty-print: Fix boost reference

* gnu/packages/pretty-print.scm (source-highlight): Change "boost-1.54" input
  to "boost"

gnu: Add a2ps, trueprint, enscript, and source-highlight

* gnu/packages/pretty-print.scm: New file
* gnu/packages/patches/source-highlight-regexrange-test.patch: New file
* gnu-system.am (dist_patch_DATA): Add patch.
(GNU_SYSTEM_MODULES): Add pretty-print.scm