directory to avoid the need to rename slang.h.
* Makefile.am: Adjust file list. Add "-Iinclude" to the
compiler flags.
* include/jdmacros.h: Rewrite to use glib.
* slmisc.c: Trim to include only needed functions.
* slang-mc.h: Add protection against using with system installed
S-Lang library.
* sl*.c: Only include _slang.h, but not slang.h to avoid making
symlinks.