Avoid using the full path and instead rely on PATH being correctly set
up to find the executable.
This fixes byte compilation for python2.7 when doing flatpak module
builds where python2.7 can be in either /usr/bin or /app/bin, depending
on how it's compiled.
The Python compileall2 module in /usr/lib/rpm/redhat/
can be executed by various different Python interpreters.
We don't want to write several different `*.pyc` files
to this location - in most cases, that's not possible,
but somebody might run this as root.
Resolves: rhbz#1595265
The problem this change is intended to solve is with how `real_libdir`
is calculated. Let's assume we want to recursively byte-compile all
`*.py` files in
`/builddir/build/BUILDROOT/python-scales-1.0.9-250.fc32.x86_64/usr/lib/python3.8`.
Then, `real_libdir` is this path without `$RPM_BUILD_ROOT` with
the filename at the end which displays in the error message like this:
```
Bytecompiling .py files below /builddir/build/BUILDROOT/python-scales-1.0.9-250.fc32.x86_64/usr/lib/python3.8 using /usr/bin/python3.8
*** Error compiling '/builddir/build/BUILDROOT/python-scales-1.0.9-250.fc32.x86_64/usr/lib/python3.8/site-packages/greplin/bar.py'...
File "/usr/lib/python3.8/bar.py", line 1
import sin from math
^
SyntaxError: invalid syntax
```
`/usr/lib/python3.8/bar.py` is obviously wrong.
One of the new features of the `compileall2` module (which will
be available in stdlib in Python 3.9) is that the path byte-compiled to
`*.pyc` files is calculated for each file. This means that by using
`-s` and `-p` we can strip `$RPM_BUILD_ROOT` and prepend `/` for each
file individually which will fix the problem.
```
Bytecompiling .py files below /builddir/build/BUILDROOT/python-scales-1.0.9-250.fc32.x86_64/usr/lib/python3.8 using /usr/bin/python3.8
*** Error compiling '/builddir/build/BUILDROOT/python-scales-1.0.9-250.fc32.x86_64/usr/lib/python3.8/site-packages/greplin/bar.py'...
File "/usr/lib/python3.8/site-packages/greplin/bar.py", line 1
import sin from math
^
SyntaxError: invalid syntax
```
This change has an effect only for Python >= 3.4.
