Use a python variable instead of hardcoding Python

The nodejs build system uses python in a number of locations. However,
there are some locations where it hardcodes 'python' as the Python
interpreter. However, this causes problems when we need to use python2
instead of just python.

This patch fixes that by using the python variable already in place in
the nodejs build system.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/deps/v8/tools/gyp/v8.gyp
===================================================================
--- a/deps/v8/tools/gyp/v8.gyp
+++ b/deps/v8/tools/gyp/v8.gyp
@@ -792,7 +792,7 @@
                 '<(SHARED_INTERMEDIATE_DIR)/libraries.cc',
               ],
               'action': [
-                'python',
+                '<(python)',
                 '../../tools/js2c.py',
                 '<@(_outputs)',
                 'CORE',
@@ -810,7 +810,7 @@
                 '<(SHARED_INTERMEDIATE_DIR)/experimental-libraries.cc',
               ],
               'action': [
-                'python',
+                '<(python)',
                 '../../tools/js2c.py',
                 '<@(_outputs)',
                 'EXPERIMENTAL',
@@ -840,7 +840,7 @@
                   '<(SHARED_INTERMEDIATE_DIR)/debug-support.cc',
                 ],
                 'action': [
-                  'python',
+                  '<(python)',
                   '../../tools/gen-postmortem-metadata.py',
                   '<@(_outputs)',
                   '<@(heapobject_files)'