gdb build script: Make the script work with non-absolute base paths

There are two problems when passing a non-absolute path to the -b flag:

1. It ends up being used in "./configure --prefix=${base_dir}".
   configure fails, because the prefix must be absolute.
2. It ends up being used to set up symlinks, leading to broken links.

Making the install path absolute fixes those.

Change-Id: I7c1d22754c1a788f02cd912f2114cc091951b303
Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
1 file changed