build: Get rid of #ifdef G_ENABLE_DEBUG

It just accumulates code that nobody ever checks compiles, and which
therefore has every chance of not compiling the day someone wants to run
it. It's also more practical to use environment variables to
enable/disable debugging output or program abort on warnings. As for
assertions, they have their own disabling mechanism when
G_DISABLE_CHECKS is set, i.e. when --disable-debug is used at our level.

Fixes: #280