sys-devel/gdb: enable verbose gdb build, bug #695936

sys-devel/gdb: enable verbose gdb build, bug #695936

While tracing missing tinfo detection Victor noticed
lack of precise arguments to gdb's linker and compiler
commands.

Two issues fixed here:
- restore default V=1 build in custome Makefile snippet
- set --disable-dependency-tracking to top-level ./configure
  to reach ./configure files that actually define it.
  Top-level does not and thus tricks portage's econf()
  into not passing it on.

Reported-by: Victor Mataré
Bug: https://bugs.gentoo.org/695936
Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>