Build: GNUmakefile: ensure build directory is defined

765716cd attempted to support VPATH builds for the GNUmakefile coverity
targets, by putting artifacts in $(builddir) and $(abs_builddir).

However, GNUmakefile may be used before configure, so those won't be defined in
that case. GNUmakefile does include Makefile if present, so the variables are
available if configure has been run. To handle both cases, define abs_builddir
to the current working directory if it isn't already defined.

In practice, VPATH builds aren't practical for GNUmakefile targets, but there's
no harm in taking some prepatory steps.