configure: automatically pick python3 is available

Unless overridden via an env var or configure arg, QEMU will only look
for the 'python' binary in $PATH. This is unhelpful on distros which
are only shipping Python 3.x (eg Fedora) in their default install as,
if they comply with PEP 394, the bare 'python' binary won't exist.

This changes configure so that by default it will search for all three
common python binaries, preferring to find Python 3.x versions.

Backports commit faf441429adfe5767be52c5dcdb8bc03161d064f from qemu

qemu/configure

@@ -484,7 +484,18 @@ Linux)
 esac
 
 : ${make=${MAKE-make}}
-: ${python=${PYTHON-python}}
+# We prefer python 3.x. A bare 'python' is traditionally
+# python 2.x, but some distros have it as python 3.x, so
+# we check that before python2
+python=
+for binary in "${PYTHON-python3}" python python2
+do
+    if has "$binary"
+    then
+        python="$binary"
+        break
+    fi
+done
 
 # Default objcc to clang if available, otherwise use CC
 if has clang; then
@@ -579,8 +590,9 @@ done
 # Remove old dependency files to make sure that they get properly regenerated
 rm -f *-config-devices.mak.d
 
-if ! has $python; then
-  error_exit "Python not found. Use --python=/path/to/python"
+if test -z "$python"
+then
+    error_exit "Python not found. Use --python=/path/to/python"
 fi
 
 # Note that if the Python conditional here evaluates True we will exit