Cherry-pick more upstream fixes to support building against python 3.8 (

#203)

* Add Python 3.8 compatibility

From Python 3.8 and onwards C extensions are no longer
linked to libpython so in order to embed python within
an application the --embed flag needs to be added to
python3-config --libs. A fall back is provided as the
command will error out on previous python versions.

References: https://bugs.python.org/issue36721

* Fix linking with Python 3.8

Since Python 3.8 --embed flag needs to be provided to python-config to embed python.
Reference: https://bugs.python.org/issue36721

This was partially addressed in collectd#3170 but only fixed building and not linking with python.

Co-authored-by: Charalampos Stratakis <[email protected]>
Co-authored-by: Alexander Meshcheryakov <[email protected]>

configure.ac

@@ -4713,11 +4713,11 @@ if test "$PYTHON3_CONFIG" != ""; then
  if test $? -ne 0; then
  with_libpython3="no"
  fi
- LIBPYTHON3_LDFLAGS="`${PYTHON3_CONFIG} --ldflags`"
+ LIBPYTHON3_LDFLAGS="`${PYTHON3_CONFIG} --ldflags --embed`" || LIBPYTHON3_LDFLAGS="`${PYTHON3_CONFIG} --ldflags`"
  if test $? -ne 0; then
  with_libpython3="no"
  fi
- LIBPYTHON3_LIBS="`${PYTHON3_CONFIG} --libs`"
+ LIBPYTHON3_LIBS="`${PYTHON3_CONFIG} --libs --embed`" || LIBPYTHON3_LIBS="`${PYTHON3_CONFIG} --libs`"
  if test $? -ne 0; then
  with_libpython3="no"
  fi