Use the same Python interpreter for the genapi.py script as for the Waf script

This ensures we call the genapi.py script with the same Python interpreter as the Waf
build system was called. This is relevant if the used interpreter is not 'python' but e.g.
python2 or python2.6 or whatever.

wscript

@@ -319,7 +319,7 @@ def build(bld):
         name    = 'geanyfunctions.h',
         before  = ['c', 'cxx'],
         cwd     = '%s/plugins' % bld.path.abspath(),
-        rule    = 'python genapi.py -q')
+        rule    = '%s genapi.py -q' % sys.executable)
 
     # Plugins
     if bld.env['HAVE_PLUGINS'] == 1: