* builds/freetype.mk (DOC_DIR): New variable.
(refdoc): Use *_DIR variables.
(distclean): Remove documentation files.

* builds/detect.mk (std_setup, dos_setup): Mention `make refdoc'.

* configure: Set DOC_DIR variable.

* builds/freetype.mk (refdoc): New target to build the
documentation.
(.PHONY): Updated.

* include/freetype/freetype.h: Improve documentation of FT_CharMap.
* include/freetype/ftimage,h: Fix documentation of FT_OUTLINE_FLAGS.
* include/freetype/tttables.h: Document FT_Sfnt_Tag.