Remove support for MinGW and MSYS.

They are outdated and problematic, and compilation was not working with them anyway (missing icons in resulting binary and failures to set the application name and such in the makefiles). Examples of MinGW/MSYS problems for which the workarounds are being removed:

* No default root certificates for wget.
* No support for Perl-style regex in grep.
* No support for Windows paths in readlink.
* No support for process substitution in bash.
* Visual styles unavailable without defining _WIN32_WINNT.
* No package for intltool.