build: default to --enable-gcc-warnings for git tree

build: default to --enable-gcc-warnings for git tree

Anyone developing on coreutils can be assumed to have a new enough
environment, such that enabling gcc warnings by default will be
useful.  Tarballs still default to no warnings, and the defaults
can still be overridden with --disable-gcc-warnings.

* configure.ac (gl_gcc_warnings): Set default based on environment.
Suggested by Bernhard Voelker.