lang/python3?: Fix python3.?-config behavior when symlinked

When python3.?-config is symlinked to another location it starts
outputting bogus paths. For example

$ pwd
/home/tobias
$ python3.6-config --includes
-I/usr/local/include/python3.6m -I/usr/local/include/python3.6m
$ ln -s /usr/local/bin/python3.6-config python3-config
$ ./python3-config --includes
-I/home/include/python3.6m -I/home/include/python3.6m

This breaks ports trying to use BINARY_ALIAS together with
python3.?-config. Apply a patch to resolve the symlink first before
trying to find the install prefix.

PR: 229749
Submitted by: tobik
Reviewed by: antoine, miwi
Approved by: python (miwi)