libslax-0.19.0

• Fix issue with libslax extensions being incorrectly symlinked under a cygwin build
• add gcc test; don't invoke BISON twice
• add linenum debug output
• add missing free calls
• allow SLAX_EXTDIR environment variable to override the builtin extension library
• avoid clang warning with slaxEltArgCallPref (by making it non-static; ick)
• avoid insanely odd gcc-on-mavericks warning when too much auto data is on the stack (ld: warning: could not create compact unwind for _slaxExtDampen: stack subq instruction is too different from dwarf stack size)
• distinquish '+' for YANG
• drop slaxconfig.h from slaxinc_HEADERS
• fix URLs for new oxtradoc
• fix line numbers
• fix line numbers; start committing yang code
• fixes for clang and gcc49, and for mavericks
• make URLs into erefs
• make anchor when none are given
• make unused intermediate nodes
• move from old gcc structure field assignments to modern new ones
• new "!! list-sections" feature
• start committing yang code
• teach oxtradoc to handle "!!include-file foo.txt"
• update test cases