bug#51225: Possible bug in python.m4, automake 1.16.4+

[Karl Berry]

https://git.savannah.gnu.org/cgit/automake.git/commit/m4/python.m4?id=ed8daa069a6c8ed34f7856c42402ec46f305e670

    However, this change leads to pkg-config .pc files containing the
    variable name $PYTHON_PREFIX, not its content

I admit I didn't test much explicitly, but my understanding was that the
use of ${PYTHON_PREFIX} in various places instead of its value was
intentional, to allow for overriding at make time.

Thus, whatever the process is for creating .pc files, I guess it
may need to change. A previous setting in the Makefile.am should yield
the value of PYTHON_PREFIX to be substituted.

? --thanks for the report, karl.