MSYS2 fixes

[nulano]

Changes proposed in this pull request:

• Remove old include directory C:\\msys64\\mingw32\\include\\libimagequant.

  It uses a hardcoded path to MSYS2, it is hardcoded to 32-bit, and it is no longer necessary, as the include is now in C:\\msys64\\mingw32\\include\\libimagequant.h.

• Remove GCC workaround for MSYS, see Replace distutils #4796 (comment)

  The issue was that AppVeyor was missing the MSYSTEM=MINGW32 envvar, and getting confused by the default MSYSTEM=MSYS2.

• Fix LibTiff support. The issue is that MSYS2 builds LibTiff with USE_WIN32_FILEIO, which changes the meaning of the fd parameter for TIFFFdOpen, see OSError: -2 when decoding a tiff_lzw file #4237 (comment). I've added a workaround by converting the parameter before passing it to LibTiff if building on MinGW.

  It might be possible to detect the system version of LibTiff by including the tif_config.h header to fix OSError: -2 when decoding a tiff_lzw file #4237 (by enabling the workaround if USE_WIN32_FILEIO was enabled), but it appears to be a private header (while it is available in MSYS2, it is not available in Ubuntu). See also passing build using CMake on Windows from my branch nulano/Pillow@mingw-setup...nulano:tiff-test.

  Another possible fix would involve absorbing TIFFFdOpen from tif_unix.c into Pillow to avoid this issue by using the stable TiffClientOpen.

• Document installation on MSYS2

[radarhere]

Suggested change

	mingw-w64-x86_64-libjpeg-turbo \
	mingw-w64-x86_64-zlib \
	mingw-w64-x86_64-libtiff \
	mingw-w64-x86_64-freetype \
	mingw-w64-x86_64-lcms2 \
	mingw-w64-x86_64-libwebp \
	mingw-w64-x86_64-openjpeg2 \
	mingw-w64-x86_64-libimagequant \
	mingw-w64-x86_64-libraqm
	mingw-w64-x86_64-freetype \
	mingw-w64-x86_64-lcms2 \
	mingw-w64-x86_64-libimagequant \
	mingw-w64-x86_64-libjpeg-turbo \
	mingw-w64-x86_64-libraqm
	mingw-w64-x86_64-libtiff \
	mingw-w64-x86_64-libwebp \
	mingw-w64-x86_64-openjpeg2 \
	mingw-w64-x86_64-zlib \

Sorted

[nulano]

The order is the same as in the External Libraries section above. I think it is more helpful to follow that order for easier cross-referencing if someone doesn't want a full install.

[lazka]

You could use https://github.com/marketplace/actions/setup-msys2

[hugovk]

Thanks!