build: use proper python interpreter

Make configure start gyp with the same python interpreter that is used to
run configure itself.

Fixes an issue where configure fails with a SyntaxError because the user
has multiple python binaries on his $PATH and the default one is too old.

Makefile

@@ -31,7 +31,7 @@ out/Debug/node:
 	$(MAKE) -C out BUILDTYPE=Debug
 
 out/Makefile: common.gypi deps/uv/uv.gyp deps/http_parser/http_parser.gyp deps/zlib/zlib.gyp deps/v8/build/common.gypi deps/v8/tools/gyp/v8.gyp node.gyp config.gypi
-	tools/gyp_node -f make
+	$(PYTHON) tools/gyp_node -f make
 
 install: all
 	out/Release/node tools/installer.js install $(DESTDIR)

configure

@@ -400,7 +400,8 @@ write('config.mk', "# Do not edit. Generated by the configure script.\n" +
   ("BUILDTYPE=%s\n" % ('Debug' if options.debug else 'Release')))
 
 if os.name == 'nt':
-  subprocess.call(['python', 'tools/gyp_node', '-f', 'msvs',
-                                               '-G', 'msvs_version=2010'])
+  gyp_args = ['-f', 'msvs', '-G', 'msvs_version=2010']
 else:
-  subprocess.call(['tools/gyp_node', '-f', 'make'])
+  gyp_args = ['-f', 'make']
+
+subprocess.call([sys.executable, 'tools/gyp_node'] + gyp_args)