gh-78292: Improve python-config build script to respect symbolic link.

[farhaanbukhsh]

Description

This PR uses realpath so that we can get the actual path of the repository even if it is a symbolic link. This way the python-config script will produce the right paths even when it is linked to a different location.

Testing instructions

These are manual testing instructions that I followed to test the PR.

Without the changes in the PR:

1. ./configure --with-pydebug
2. make python-config

mkdir -p /tmp/cpython-repro/bin

cp python-config /tmp/cpython-repro/bin/python3-config
chmod +x /tmp/cpython-repro/bin/python3-config

5. /tmp/cpython-repro/bin/python3-config --prefix will produce output /tmp/cpython-repro
6. mkdir -p /tmp/elsewhere/bin
7. ln -s /tmp/cpython-repro/bin/python3-config /tmp/elsewhere/bin/python3-config
8. /tmp/elsewhere/bin/python3-config --prefix this show produce the output /tmp/elsewhere which is the bug
9. Once you check out the PR and repeat the above steps the command /tmp/elsewhere/bin/python3-config --prefix will produce /tmp/cpython-repro, which is expected.

• Issue: python-config breaks when symlinked to another location #78292