Switch to gettext 0.23 (#39)

* Switch to gettext 0.23

* CLDR plurals in shared builds works with gettext 0.23

 Support Windows 7+ instead of Windows 10+ (#36)

* Move -g0 -O2 from CPPFLAGS to CFLAGS/CXXFLAGS

That's the more correct way to pass such options to the compiler

* Support Windows 7+ instead of Windows 10+

Update .po files from translationproject.org (#27)

* Update .po files from translationproject.org

* Install gettext-develop with apt-cyg

Installing it with Cygwin installer installs also a ton of other stuff

* Rebuild .gmo files too

Fix typo in README.md

Recognize ws2_32.dll as a system library

gettext 0.20.2

Add helper to list required DLLs

Simplify setup building instructions

iconv 1.15

Patch for gnulib bug

See http://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=68b6adebef05670a312fb92b05e7bd089d2ed43a