gdb: working with debug symbols

[ruslo]

Migrated from https://github.com/ruslo/hunter/issues/359#issuecomment-240917420:

gdb would require a .gdbinit file in the debugging directory with:

set substitute-path <old-build-directory>/Build <root-id>/Install

Source: https://sourceware.org/gdb/onlinedocs/gdb/Source-Path.html