Correct binary name to search for both windows and posix

tools/build/premake

@@ -54,6 +54,7 @@ setup_premake_path_override()
 
 
 def main():
+  premake_bin_name = 'premake5.exe' if sys.platform is 'win32' else 'premake 5'
   premake_build_bin = os.path.join(premake_path, 'bin', 'release', 'premake5')
   premake_local_bin = os.path.join(self_path, 'bin', 'premake5')
   premake5_bin = None
@@ -81,10 +82,6 @@ def main():
       print('ERROR: cannot build premake5 executable.')
       sys.exit(1)
 
-  if sys.platform == 'win32':
-    # Append the executable extension on windows.
-    premake5_bin = premake5_bin + '.exe'
-
   return_code = shell_call([
       premake5_bin,
       '--scripts=%s' % (premake_path),