scripts/env_use_destdir: fix Fedora support

libselinux and libsemanage use:

    PYTHONLIBDIR ?= $(shell $(PYTHON) -c "from distutils.sysconfig
    import *; print(get_python_lib(plat_specific=1,
    prefix='$(PREFIX)'))")

while python/semanage and python/sepolgen/src/sepolgen use:

    PYTHONLIBDIR ?= $(shell $(PYTHON) -c "from distutils.sysconfig
    import *; print(get_python_lib(prefix='$(PREFIX)'))")

This is right: libselinux and libsemanage's Python bindings use native
code (thus "plat_specific=1") while the others only install Python
files.

Nevertheless `scripts/env_use_destdir` only runs the second command
when computing `$PYTHONPATH`. When using this script to run `make test`
in a minimal Fedora 31 environment, this leads to an error such as:

    make[2]: Entering directory '/code/python/sepolicy'
    Traceback (most recent call last):
      File "test_sepolicy.py", line 117, in <module>
        import selinux
    ModuleNotFoundError: No module named 'selinux'

Fix this by also adding `get_python_lib(plat_specific=1)` to the
computed `$PYTHONPATH`.

While at it, preserve `$PYTHONPATH` instead of resetting it. This makes
it easier to work with Python virtual environments.

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>

scripts/env_use_destdir

@@ -24,8 +24,14 @@ fi
 export LD_LIBRARY_PATH="$DESTDIR/usr/lib:$DESTDIR/lib"
 export PATH="$DESTDIR/usr/sbin:$DESTDIR/usr/bin:$DESTDIR/sbin:$DESTDIR/bin:$PATH"
 
-# shellcheck disable=SC2155
-export PYTHONPATH="$DESTDIR$(${PYTHON:-python3} -c "from distutils.sysconfig import *;print(get_python_lib(prefix='/usr'))")"
+NEW_PYTHONPATH="$DESTDIR$(${PYTHON:-python3} -c "from distutils.sysconfig import *;print(get_python_lib(plat_specific=1, prefix='/usr'))"):$DESTDIR$(${PYTHON:-python3} -c "from distutils.sysconfig import *;print(get_python_lib(prefix='/usr'))")"
+if [ -n "${PYTHONPATH:-}" ] ; then
+    # Prefix the PYTHONPATH with the new directories
+    export PYTHONPATH="$NEW_PYTHONPATH:$PYTHONPATH"
+else
+    # Define PYTHONPATH
+    export PYTHONPATH="$NEW_PYTHONPATH"
+fi
 
 # shellcheck disable=SC2155
 export RUBYLIB="$DESTDIR/$(${RUBY:-ruby} -e 'puts RbConfig::CONFIG["vendorlibdir"]'):$DESTDIR/$(${RUBY:-ruby} -e 'puts RbConfig::CONFIG["vendorarchdir"]')"