* Replace `--enable-gtk3` with `--enable-gtk2`
* Update Travis CI builds
* Change `cross-build-mingw.sh` default to GTK3 and cleanup a little
After this commit it will be required to pass `--enable-gtk2` to
the Autoconf script in order to build with GTK+2, otherwise GTK+3
will be required.
* Update Scintilla to version 3.7.5
This now requires a C++11-capable compiler.
Closes#1308.
* Test using newer dist on Travis
Since Scintilla needs C++11
* Add debugging code for when configure fails
* Workaround a pkg-config-corsswrapper bug on Ubuntu 14.04
See https://bugs.launchpad.net/ubuntu/+source/mingw-w64/+bug/1327242