Making ftgrays.c compile stand-alone again.

* include/freetype/ftimage.h: Include ft2build.h only if _STANDALONE_
isn't defined.
* src/smooth/ftgrays.c [_STANDALONE_]: Define ft_memset,
FT_BEGIN_HEADER, FT_END_HEADER.
(FT_MEM_ZERO): Define.
(TRaster) [GRAYS_USE_GAMMA]: Use `unsigned char' instead of FT_Byte.
(gray_render_span, gray_init_gamma): Don't use `FT_UInt'.
Don't cast with `FT_Byte'.
(grays_init_gamma): Don't use `FT_UInt'.

* src/base/ftinit.c (FT_Add_Default_Modules): Improve error message.
* src/pcf/pcfdriver.c (PCF_Face_Done): Improve tracing message.
* include/freetype/config/ftoption.h (FT_MAX_MODULES): Increased
to 32.