* 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
Merge branch 'develop' into configure
Fix a segmentation fault issue reported by @arakiken
Merge branch 'develop' into sandbox
Merge branch 'fix-segfault-in-stbi_image' into arakiken
Check each of gdImageCreateFromXXX availabilities
Compile loader module without missing GD symbols
Check host environment
Check libiconv avilability if --with-gd is specified
Merge branch 'configure-support-less-featured-gd' into cygwin
Merge branch 'fix-for-cygwin-environment' into cygwin
Check standard jpeg loader with GD integration
Use standard jpeg loader if available
Rebuild
Merge branch 'configure-support-less-featured-gd' into cygwin