build: default to --enable-gcc-warnings in a git tree

build: default to --enable-gcc-warnings in a git tree

Anyone building from cloned sources 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 default
can still be overridden with --disable-gcc-warnings.
* configure.ac (gl_gcc_warnings): Set default based on environment.