configure: more resilient Python version capture

The current approach to capture the Python version is fragile, as it
was demonstrated by a very specific build of Python 3 on Fedora 29
that, under non-interactive shells would print multiline version
information.

The (badly) stripped version output would be sent to config-host.mak,
producing bad syntax and rendering the makefiles unusable.  Now, the
Python versions is printed by configure, but only a simple (and better
controlled variable) indicating whether the build system is using
Python 2 is kept on config-host.mak.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

Message-Id: <20190826155832.17427-1-crosa@redhat.com>
Reviewed-by: NTony Nguyen <tony.nguyen@bt.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

configure

@@ -1864,7 +1864,7 @@ if ! $python -c 'import sys; sys.exit(sys.version_info < (2,7))'; then
 fi
 
 # Preserve python version since some functionality is dependent on it
-python_version=$($python -V 2>&1 | sed -e 's/Python\ //')
+python_version=$($python -c 'import sys; print("%d.%d.%d" % (sys.version_info[0], sys.version_info[1], sys.version_info[2]))' 2>/dev/null)
 
 # Suppress writing compiled files
 python="$python -B"
@@ -6511,6 +6511,7 @@ if ! $python -c 'import sys; sys.exit(sys.version_info < (3,0))'; then
   echo
   echo "warning: Python 2 support is deprecated" >&2
   echo "warning: Python 3 will be required for building future versions of QEMU" >&2
+  python2="y"
 fi
 
 config_host_mak="config-host.mak"
@@ -7333,7 +7334,7 @@ echo "INSTALL_DATA=$install -c -m 0644" >> $config_host_mak
 echo "INSTALL_PROG=$install -c -m 0755" >> $config_host_mak
 echo "INSTALL_LIB=$install -c -m 0644" >> $config_host_mak
 echo "PYTHON=$python" >> $config_host_mak
-echo "PYTHON_VERSION=$python_version" >> $config_host_mak
+echo "PYTHON2=$python2" >> $config_host_mak
 echo "CC=$cc" >> $config_host_mak
 if $iasl -h > /dev/null 2>&1; then
   echo "IASL=$iasl" >> $config_host_mak

tests/Makefile.include

@@ -1135,7 +1135,7 @@ TESTS_RESULTS_DIR=$(BUILD_DIR)/tests/results
 AVOCADO_SHOW=app
 AVOCADO_TAGS=$(patsubst %-softmmu,-t arch:%, $(filter %-softmmu,$(TARGET_DIRS)))
 
-ifneq ($(findstring v2,"v$(PYTHON_VERSION)"),v2)
+ifneq ($(PYTHON2),y)
 $(TESTS_VENV_DIR): $(TESTS_VENV_REQ)
 	$(call quiet-command, \
             $(PYTHON) -m venv --system-site-packages $@, \