* img2sixel.c: Use gdk-pixbuf and libcurl.
* build-gdkpixbuf.sh: Added.
this commit is imported from arakiken/libsixel.
https://bitbucket.org/arakiken/libsixel/commits/fda0bcab1ec2598bada3d102d8f0cf35d848f6ff
Merge branch 'arakiken-sandbox' into arakiken

Conflicts:
	converters/img2sixel.c
Merge branch 'develop' into arakiken
Fix build
Merge branch 'fix-if-pkgconfig-is-unavilable' into arakiken
Merge branch 'issue-2' into arakiken
Fix a segmentation fault issue reported by @arakiken
Merge branch 'fix-segfault-in-stbi_image' into arakiken
Add some private header files to the source distribution
Rebuild Makefile.in
Merge branch 'configure' into build
Merge branch 'cygwin' into build
Merge branch 'build' into develop
Merge branch 'arakiken' into develop
Add an additional test image
Update README
Merge branch 'develop' into sandbox
Drop unused build script build-gdkpixbuf.sh
Merge branch 'arakiken' into configure
Update configure script