Fix linking with Python 3.8

The behaviour of python-config --libs has changed in Python 3.8.

For example, compare the output of it with Python 3.7 and 3.8 on an
ArchLinux system:

    $ python3.7-config --libs
    -lpython3.7m -lcrypt -lpthread -ldl  -lutil -lm
    $ python3.8-config --libs
    -lcrypt -lpthread -ldl  -lutil -lm -lm
    $

The lack of -lpython in the latter case causes the linker to fail when
attempting to build Yosys against Python 3.8.

Passing the new --embed flag to python-config adds -lpython, just like
earlier versions of Python:

    $ python3.8-config --embed --libs
    -lpython3.8 -lcrypt -lpthread -ldl  -lutil -lm -lm
    $

This commit adds code for automatically detecting support for the
--embed flag. If it is supported, it is passed to all python-config
invocations. This fixes building against Python 3.8.

Author: grahamedgecombe

Makefile

@@ -152,7 +152,14 @@ ifeq ($(ENABLE_PYOSYS),1)
 PYTHON_VERSION_TESTCODE := "import sys;t='{v[0]}.{v[1]}'.format(v=list(sys.version_info[:2]));print(t)"
 PYTHON_VERSION := $(shell $(PYTHON_EXECUTABLE) -c ""$(PYTHON_VERSION_TESTCODE)"")
 PYTHON_MAJOR_VERSION := $(shell echo $(PYTHON_VERSION) | cut -f1 -d.)
+
+ENABLE_PYTHON_CONFIG_EMBED ?= $(shell $(PYTHON_EXECUTABLE)-config --embed --libs > /dev/null && echo 1)
+ifeq ($(ENABLE_PYTHON_CONFIG_EMBED),1)
+PYTHON_CONFIG := $(PYTHON_EXECUTABLE)-config --embed
+else
 PYTHON_CONFIG := $(PYTHON_EXECUTABLE)-config
+endif
+
 PYTHON_PREFIX := $(shell $(PYTHON_CONFIG) --prefix)
 PYTHON_DESTDIR := $(PYTHON_PREFIX)/lib/python$(PYTHON_VERSION)/site-packages