autogen.sh: Detect python

It helps to avoid an error on distros which has only python3 binary:
  ./autogen.sh: line 20: python: command not found

Use python3 as the default as python2 is EOL since Jan 2020. However,
check also for python which is on most distros, if not all, python2
because code still works on python2.

Although it should not be needed keep the possibility to define PYTHON
variable.

For detection use "command -v" which is POSIX and supported on all
common shells (bash, zsh, dash, busybox sh, mksh) instead requiring
"which" as an extra dependency (usable on containers).

Update the INSTALL file too.

Signed-off-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

autogen.sh

@@ -7,8 +7,21 @@ if [ ! -e grub-core/lib/gnulib/stdlib.in.h ]; then
   exit 1
 fi
 
-# Set ${PYTHON} to plain 'python' if not set already
-: ${PYTHON:=python}
+# Detect python
+if [ -z "$PYTHON" ]; then
+  for i in python3 python; do
+    if command -v "$i" > /dev/null 2>&1; then
+      PYTHON="$i"
+      echo "Using $PYTHON..."
+      break
+    fi
+  done
+
+  if [ -z "$PYTHON" ]; then
+    echo "python not found." >&2
+    exit 1
+  fi
+fi
 
 export LC_COLLATE=C
 unset LC_ALL