autogen.sh portability fixes

- Do not assume tar has --version, as BSD tar does not
- Allow specifying python binary through PYTHONBIN in case it is e.g. python2.7

BUG: 764655
Change-Id: I71f0f4830e10915782775de811c92db8e6ab4c55
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/6281
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>

autogen.sh

@@ -55,11 +55,20 @@ else
 fi
 
 # Check for tar
-env tar --version > /dev/null 2>&1
+env tar -cf /dev/null /dev/null > /dev/null 2>&1
 if [ $? -ne 0 ]; then
   MISSING="$MISSING tar"
 fi
 
+# Check for python
+if [ "x${PYTHONBIN}" = "x" ]; then
+  PYTHONBIN=python
+fi
+env ${PYTHONBIN} -V > /dev/null 2>&1 
+if [ $? -ne 0 ]; then
+  MISSING="$MISSING python"
+fi
+
 ## If dependencies are missing, warn the user and abort
 if [ "x$MISSING" != "x" ]; then
   echo "Aborting."
@@ -77,7 +86,7 @@ fi
 
 ## generate gf-error-codes.h from error-codes.json
 echo "Generate gf-error-codes.h ..."
-if ./gen-headers.py; then
+if ${PYTHONBIN} ./gen-headers.py; then
     if ! mv -fv gf-error-codes.h libglusterfs/src/gf-error-codes.h; then
 	exit 1
     fi