* 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 'travis' into configure-gdkpixbuf
Merge branch 'develop' into release-0.19
Update minor version
Update configure script
Merge branch 'release-0.19'
Merge branch 'develop'
Minor fix
Merge branch 'sandbox' into configure

Conflicts:
	configure.ac
Update manpage of img2sixel
Merge branch 'arakiken-sandbox' into arakiken

Conflicts:
	converters/img2sixel.c
Merge branch 'develop' into arakiken
Fix build
Fix for environments that do not have pkg-config
Improve configure script avilability
Update build scripts
Merge branch 'fix-if-pkgconfig-is-unavilable' into configure
Merge branch 'fix-if-pkgconfig-is-unavilable' into arakiken
Merge branch 'configure' into kmiya
Merge branch 'kmiya' into sandbox