build.cmd: add support for Ninja

The patch does three changes:

1) Makes build.cmd switch between "Ninja" and "MinGW Makefiles"
   generators depending on the presence of Ninja in the system.

2) Makes build.cmd detect ninja.exe installed outside the MinGW
   environment (which is the case of standalone MinGW installation)

3) Adds a '--dev' option to build.cmd. This option enables
   "CodeBlocks" generator (so that QtCreator could use this build)
   and -DBUILD_TESTING=ON for unittests.

CC:kimageshop@kde.org