RSB fails to build gdb in virtual environment

[Sebastian Huber]

If you work in a virtual Python environment:

/tmp > python3 -m venv env
/tmp > ll env/bin/
total 32
-rw-r--r-- 1 sebastian_h domain users 2182 Aug  4 19:56 activate
-rw-r--r-- 1 sebastian_h domain users 1238 Aug  4 19:56 activate.csh
-rw-r--r-- 1 sebastian_h domain users 2402 Aug  4 19:56 activate.fish
-rwxr-xr-x 1 sebastian_h domain users  234 Aug  4 19:56 easy_install
-rwxr-xr-x 1 sebastian_h domain users  234 Aug  4 19:56 easy_install-3.6
-rwxr-xr-x 1 sebastian_h domain users  225 Aug  4 19:56 pip
-rwxr-xr-x 1 sebastian_h domain users  225 Aug  4 19:56 pip3
-rwxr-xr-x 1 sebastian_h domain users  225 Aug  4 19:56 pip3.6
lrwxrwxrwx 1 sebastian_h domain users    7 Aug  4 19:56 python -> python3
lrwxrwxrwx 1 sebastian_h domain users   16 Aug  4 19:56 python3 -> /usr/bin/python3
/tmp > . env/bin/activate
(env) /tmp > which python3
/tmp/env/bin/python3

then python3-config is not mapped with a symbolic link. This breaks the Python stuff in source-builder/config/gdb-common-1.cfg because it wants to call /tmp/env/bin/python3-config which does not exist.